Daniil Kvyat reckons it is "alarming" that McLaren was able to fight so closely with his Toro Rosso team during Formula 1's Italian Grand Prix.
Kvyat spent a significant part of the race locked in a fight with McLaren's Stoffel Vandoorne before the Belgian was forced to retire on lap 33 with a problem.
Kvyat went on to finish 12th, a lap down, after one of the most difficult races of the year yet for Toro Rosso.
The Russian reckons McLaren, which has struggled with the power of its Honda engine all year, had a quicker car at Monza.
"Even McLaren was faster than us today," said Kvyat, whose team is still 29 points ahead of McLaren in the standings. "They were looking very strong.
"They were focusing more on the corners with their set-up and it was much harder for him [Vandoorne], but at the Parabolica he was able to stay close and I had too close the line every lap.
"It wasn't easy. It's a bit alarming if McLaren can stay with us in this kind of track.
"There's a lot to do and we need to start working on developing the strong point of the car."
Team-mate Carlos Sainz Jr endured a similarly difficult day and, having started 15th, he wound up 14th in the race.
"We were running a lot less downforce than our competitors to try and compensate the power loss," Sainz said.
"We were also definitely faster than the Renault on the straights but a lot slower than them on the corners.
"If we would have started in the top 10, with our top speed, we might have just held on to it, but starting 15th, when you don't have the pace to recover, it's impossible."
The Spaniard reckons, however, that Red Bull's strong pace this weekend showed Toro Rosso can't put the blame exclusively on the Renault engine.
"Red Bull shows that even without the most competitive power unit, you can still look good in circuits like Monza," he said.
"We just didn't have a good package for coming into Monza and we must learn from it and see what's our defects and solve them for Singapore because we need to start bringing some good results just take a step forward."