Sergio Perez has raised concerns about the new Pirelli rubber after witnessing "extreme" degradation at the Circuit de Catalunya on Wednesday.
Putting in the laps at the Spanish track on Day Two of the second test, Perez posted the fastest time, a 1:21.848.
However, the 23-year-old was not able to match his P1 time on his next lap due to the severe drop-off in tyre performance.
"It's extreme. It's very, very difficult the degradation - it's a big surprise," Perez said.
"Normally we see in winter testing a lot of degradation, but never this much.
"We are going to have sometimes cold races like here, so it's a bit of a worry, but we are still learning about the tyres and once we go racing I hope things can change with the tyres.
"I hope it changes, because if we are in this situation in Melbourne we are going to see something like seven or 10 stops."
Last season Perez proved to be a master of the Pirellis, often running longer stints as he worked his way to the front of the pack. This year he fears that won't be possible.
"As a driver obviously you can help the tyres to last a bit longer, but at the moment we are learning from the tyre is basically very weak and there's very little the driver can do now to help the degradation.
"The tyre is difficult to do two or three laps with it. Once you do a lap you are starting to fight degradation so it's pretty difficult to learn anything from your car or from the balance."
Perez is relishing his first season with McLaren where he expects to be in a competitive car. However, how competitive his MP4-28 is he isn't saying.
"I think the car is definitely competitive. We have no idea how the others are, the car has a good base but this base has to be improved for the first race. The team is working very hard to find more and it has to be our approach. I think we have to find quite a bit more downforce for the first race.
"I think the car in general is quite strong, it has a very good base and a very good potential to be improved if we can make them work. It's very difficult to tell you that the high-speed is bad for us, or the low-speed [for example]. I think in general we have to improve our car."
