Pedro de la Rosa believes Bridgestone and the teams need a lot more work before they find the right slicks for 2009, the Spaniard admitting the lack of tyre warmers is a concern right now.
Tyre warmers will be banned from the 2009 season, when slick tyres will also be making a return to Formula One.
Downforce levels will be drastically reduced too, making things even trickier for the drivers.
Some drivers in the past have expressed their concerns about running with cold tyres, because of the speed difference with cars that are already up to racing pace with warmed-up rubber.
Teams struggled on their opening laps when they tested the slicks for the first time at Jerez last December, and Bridgestone has introduced new compounds for this week's second test.
On Monday, de la Rosa was running with simulated 2009 downforce levels and with slicks not previously warmed up.
The McLaren tester admitted the situation has improved since the Jerez test, but he reckons there is still a lot of work to be done in order to improve the safety aspect.
"Bridgestone is trying to make the tyres work at low temperatures, because in theory we are not going to have tyre warmers," de la Rosa told reporters in Barcelona.
"We had a lot of problems in Jerez to make them work without the tyre warmers. Here it was better. It's still difficult but it was somewhat better, because Bridgestone has created a compound that works in lower temperatures.
"The first lap is very slow, and that's the danger. There are cars which are up to racing speed and you are coming out of the pits very slowly. You are like a mobile chicane.
"That's the problem that this kind of rules has. But Bridgestone has taken a big step forward and we still need to improve a lot."
De la Rosa said he needed around two laps before the tyres were up to temperature.
The Spaniard admitted, however, that driving with such a different car was a challenging situation.
"It was interesting. Because driving a car with no electronics aids, with low downforce and slicks is very different from what I'm used to, but it was very interesting.
"We were running the downforce levels that we think we are going to get in 2009, which are very low, about 50%.
"These numbers are just predictions because you always gain a bit later. But we have been running with the wings that generate less downforce, especially to learn more with the slicks."