Has Rossi's replacement already stepped forward?

You would be hard-pressed to find too many people involved in MotoGP looking forward to Valentino Rossi's retirement, whenever that may be. It's still at least 18 months away, and might not even come when Rossi's current Yamaha contract concludes at the end of 2018.

Rossi being Rossi, you wouldn't bet against him racing beyond his 40th birthday in February 2019. So it may seem a little early to cast an eye towards his exit, especially if he maintains the form that has him leading the 2017 championship after four races.

But if this does end up being Rossi's last Yamaha MotoGP deal, whose number gets stuck on a factory M1 when the yellow #46 comes off at the end of 2018? If Yamaha had to make that decision any time soon, I reckon a red and white #5 would be the one being printed.

The silly season for 2017 and '18 factory rides was basically run and won by the end of May last year. Rossi and Bradley Smith got the ball rolling in Qatar in March, with a Yamaha extension and new KTM deal respectively.

Jorge Lorenzo announced his Ducati switch in late April, before the first European race at Jerez. By then, Maverick Vinales was having sleepness nights weighing up staying at Suzuki or replacing Lorenzo, which he ultimately settled on after taking his first podium at Le Mans.

Effectively bumped out of Ducati, Andrea Iannone's move to Suzuki was announced on the same mid-May morning as Vinales' move to Yamaha. Honda casually re-signed Marc Marquez and Dani Pedrosa, who was linked briefly with Yamaha as cover for Vinales staying put, by early June.

By the end of June, the game of factory musical chairs was over, with Pol Espargaro at KTM, Alex Rins at Suzuki and Aleix Espargaro at Aprilia, which had signed Sam Lowes for 2017 way back in September '15.

What that really meant is that riders such as Vinales in particular were largely assessed, and then signed or overlooked, based on what they had achieved in the preceding season.

What's to say we're not in a similar situation early next year, for 2019 rides, if Rossi decides to hang up the leathers and makes his intentions known in the opening races?

This year's standout performers, Vinales is already at Yamaha and Marquez would be considered unlikely to leave Honda. And, while there is not the outright animosity that has existed between Rossi and Marquez, there is a subtle intensity to the relationship between Vinales and Marquez.

Lorenzo has struggled but looked better on the Ducati at Jerez, taking his first podium. Even if he did endure a miserable run between now and Valencia 2018, surely pure stubborness would mean he wouldn't run back to Yamaha after two years, like Rossi did.

His current Ducati team-mate Andrea Dovizioso could be an option, if Yamaha wanted an experienced rider who wouldn't rock the boat, but he will be 33 by the time the 2019 season starts. Ditto sublime Jerez winner Pedrosa and Cal Crutchlow, the best of the satellite riders. And that takes you to the end of the list of multiple GP winners on the grid.

Then it's into wildcards, such as the ever-improving Jack Miller - 2016 Assen winner and in the final season of his Honda contract - and Iannone, who should probably focus on what's on his plate at Suzuki for now.

Johann Zarco, by contrast, looks about as good a fit as Yamaha will be able to find. And that's not as far-fetched as it seems. The two-time Moto2 world champion has been massively impressive on his Tech3-run Yamaha, admittedly from a sample set of only four grands prix.

I wasn't massively sold on Zarco this time last year. Yes, he dominated Moto2 in 2015, his fourth season, but the Frenchman's title defence did not inspire confidence early.

He won in Argentina but, including a crash at his home race, was a bit hit and miss. That was followed by an impressive surge, but another dip mid-season kept Rins and Lowes in contention despite their own misadventures. Zarco ultimately did the business to wrap it up with a race to spare.

Perhaps if he'd made a better start to last season, Zarco would already be a factory MotoGP rider. Suzuki had an option on him for 2017, but never really seemed outwardly convinced. His best bet to ride a Suzuki looked to be if it reached a deal to supply a satellite team, which didn't happen. Zarco tested at Motegi last June, days before Rins was announced as Iannone's partner.

Zarco's consolation prize of a Tech3 ride has probably worked out better.

It is a slightly one-sided comparison given Rins has had an unfortunate, injury-riddled start to life in MotoGP. But other than nailing a lap in third-practice in Qatar - which put him on the front row when qualifying was washed out - and running in the lead pack early there and in Spain, Iannone has been largely anonymous. If anything, it shows just how good Vinales was on the Suzuki last year.

Instead, Zarco and fellow rookie Jonas Folger have jumped onto the 2016 Yamaha that should have won more races than it did with Rossi and Lorenzo. They also have the benefit of it being designed to use Michelin tyres, while Tech3 predecessors Smith and Espargaro spent 2016 on the '15 M1 tuned around Bridgestones.

The Yamaha is, as Rossi has said, the "friend of the rider". It's the best bike to enter MotoGP on and by some margin, given the way it generates its more-than-ample pace and responds, especially compared to learning the premier class and being on a Honda or Ducati.

"The Yamaha, for a rookie, gives you a lot of confidence and it's an easy bike," Lorenzo surmised, having made his debut on one in 2008.

There's no denying that has helped Zarco and Folger make an impression. Both have shown good speed and gone about things the right way. Folger's Tech3 deal, done before Zarco's, was more heavily scrutinised but he has adapted well. While the German is playing second fiddle to Zarco for now, evidence suggests he also has a good future ahead.

Folger grabbed the headlines during the first pre-season tests, finishing fourth in the second week of running at Phillip Island. While Folger had more confidence at that time to push, Zarco was regrouping from a fall during Yamaha's private test at Sepang last November, the week after the post-season Valencia running.

It would have been easy for a rookie to let that get into their head. To try to change their own approach, lose confidence or look to do too much to get back on terms and slide down the road.

But Zarco is seriously methodical and comfortable working out - at his own pace - how to ride a MotoGP bike and get the most out of it from his smooth style. And, just as that approach worked in Moto2, it's paying off so far.

Broadly speaking, that sort of explains Zarco's career progression. At 26, turning 27 in July, he's not exactly a young rookie. By comparison, Vinales is in his third season of MotoGP at 22, having moved through the ranks at a rate that admittedly makes most riders look tardy.

Zarco finished second in the 2011 125cc championship, his third campaign, one spot ahead of rookie Vinales. Vinales stayed in what became Moto3 for two more seasons, winning the 2013 title. He then finished third as a Moto2 rookie in '14, three spots ahead of third-year Zarco. From there, Vinales spent two seasons in MotoGP before Zarco even made his premier-class debut.

Now that he's in MotoGP, though, Zarco isn't wasting any time. It's not so much that he's sixth in the championship and has finished three of his first four races in the top five, it's how he's gone about it.

He has taken no prisoners, and shown more than enough confidence to commit to attacking. From fourth on the grid on his debut in Qatar, he forced his way past the factory Hondas and then Vinales at the first corner, and by halfway around the lap had taken the lead from Iannone.

Out in front, Zarco built a lead for the first six laps, before crashing at Turn 2. At Termas de Rio Hondo he started 14th but was up to sixth by lap five. At Austin, he started on the second row again and pressured Rossi in the early stages. Zarco's early move at Turn 3 forced Rossi to the runoff and later the veteran questioned his 'Moto2 tactics'.

Zarco was not ruffled by that, and was defended by the likes of Marquez and Crutchlow. Last Sunday's race at Jerez was another clear example of how strong he is early.

One of four riders using the medium front tyre, rather than the hard, he passed Rossi twice on the first lap, then Vinales, Crutchlow and Iannone on lap two to slot into third place. Zarco got ahead of Marquez briefly but couldn't stay with the Hondas or then keep the similarly-medium-shod Lorenzo at bay to take a maiden podium, settling for fourth.

Those early results are nothing to be sneezed at, but it is how Zarco is racing that is rightfully turning heads. His moves have been clean and forceful without crossing the line. Sort of devoid of rookie exuberance, and largely successful at the first attempt. Rivals have generally been relaxed, the consensus that he is aggressive, but that you have to be.

"He reminds me a little bit of me when I arrived in MotoGP [in 2013]," Marquez said. "Really aggressive, pushing on the limit and warnings and nearly crashing, but in the end it is the way to learn."

Rossi was the most aggrieved at Austin, unhappy at how late Zarco made his move and feeling he had nowhere to go. But you only have to go back to Misano last September for an instance of Rossi being criticised for making a move that was "maybe too aggressive", on team-mate Lorenzo.

Lorenzo raced and passed Zarco at Jerez, and without citing any specific incidents, conceded he is "sometimes a little bit too determined, especially when he is fighting with another rider. But apart from that I like a lot the attitude of him and he's a really, really good rider."

That a double Moto2 champion is a "really, really good rider" is a given, but Zarco is making the most of opportunity he has. Riding and racing well.

Yamaha will be keeping an eye on what he's doing at Tech3, its de-facto junior team. You would think Zarco is impressing it, in a way that Pol Espargaro - who had a Yamaha contract - could not sufficiently to replace Lorenzo when push came to shove.

Its next big decision, potentially for 2019, will be how to fill an even bigger void when Rossi retires. If he builds on this strong start to his MotoGP career, Zarco might well make that call an easy one.

Assuming another manufacturer doesn't swoop first.