Valentino Rossi admits he is "worried" about Yamaha's engine troubles following his retirement at the Italian Grand Prix in Mugello last month.
Rossi suffered an engine failure while trailing team-mate and race-leader Jorge Lorenzo, who suffered an identical issue in the warm up.
The retirement left Rossi 37 points adrift of defending MotoGP champion Lorenzo, who crossed the line just ahead of Honda's Marc Marquez, after the first six races of the season.
Yamaha revealed the failures were caused by the way the rev limiter worked on the new standard ECU software, and Rossi admits he is slightly concerned heading into Sunday's Catalan GP.
"This year we have seven engines, two more than last year, but we have to be worried [by what happened]," the Italian said.
"But in Yamaha they are quiet and I think that more or less the bike will be the same.
"Maybe we lose some RPM. Maybe. But I'm not sure. I think that the performance will be the same - also because the other engines that have already been used have a lot of kilometres and they are okay.
"So it was something unlucky at Mugello."
The seven-time premier class champion added: "In motorsport when an engine is broken it is always like this, an electronic issue or a one-dollar part!
"It looks like the rev limiter is not very precise like last year and with the small jump that the bike does on the hill maybe the RPM got too high and broke the engine.
"It was a great shame. But I think and I hope it doesn't happen for the rest of the season."