Sunday's Qatar MotoGP signalled the start of a new Petronas Yamaha era for Valentino Rossi, but a familiar problem returned to haunt the Italian in the race.

After qualifying a strong fourth, Rossi held seventh in the early Losail laps but was soon struggling with rear tyre performance.

Overstressing the rear tyre is an issue Rossi battled frequently during his final seasons at the factory Yamaha team and on Sunday it caused him to fade back to twelfth place.

"We expected more for sure, especially after the qualifying yesterday, but in the race I struggled very much because after some laps I had problems with the tyres," Rossi explained.

"This also happened on Friday. We tried to do something but unfortunately we didn’t fix the problem. For some reason we stress too much the rear tyre and it looks like the rear tyre starts to lose performance and grip, so after some laps I'm not fast enough.

"We hoped today we would have less problems because the temperature is a bit colder, but unfortunately the feeling was similar and the race was difficult. I struggled a lot. I lost a lot of positions.

"We've had a long meeting, I said what my feelings are and now we will try to improve and fix the problem to be stronger next week."

Rossi had joined the rest of the grid in selecting soft tyres (front and rear) for the 22-lap race, having tried the hard rear in practice.

"Yesterday we tried the hard rear because on Friday we had the problems with the soft, but with the hard we were not fast enough. So we decided to race with the soft. But we faced the same problem even if the temperature was less today," he confirmed.

The race was won by Rossi's former Factory Yamaha team-mate Maverick Vinales, whose new team-mate Fabio Quartararo was fifth.

"I make a great congratulations to Maverick because he rode incredibly, because he was with me at the beginning of the race," Rossi said.

"It's positive that a Yamaha won. They have something different in the setting compared to us and it looks like it works very well because also Quartararo was very strong.

"So it's good to try to understand if we are able to improve next week."

But if Rossi was disappointed on Sunday night, spare a thought for Petronas team-mate and reigning title runner-up Franco Morbidelli.

Morbidelli came into the race as one of the favourites but plummeted from seventh to 20th within four laps, eventually finishing out of the points in 18th.

"I don’t know what happened to Franco, I think he had some problem with the bike," Rossi said.

Morbidelli later confirmed a technical issue with his A-Spec machine which made it feel as if he has no damping on the rear suspension, forcing him to ride one-second slower.

"We had a problem on the grid, but we decided to go anyway for the race," Morbidelli said.

"It is a problem that we need to investigate better because I think it happened also other times during the weekend, but in the race we saw clearly that something was wrong, and unfortunately we had to make such a bad race.

"It affected me during the whole race. I cannot tell you what it is because we are not sure yet. We are trying to understand well what it is.

"Basically, the rear-end of the bike starts to feel 'empty', like I have no damping in the shock and I have no shock. I can't turn. Well, I can turn, but I need to ride 1-second slower. I can accelerate, but I need to ride 1-second slower.

"That's the effect that I have. We don't know the cause yet. We are trying to investigate, I'm sure the Yamaha guys will help me in this matter."

Despite being "pretty much last" Morbidelli decided to ride on to the finish out of "respect for my team and the people working with me" and "to study the problem better".

Something related to the (rear lowering) ride-height device is one possibility. Another issue for Morbidelli provided was that he performed much better during the hotter daylight sessions, being fastest in both FP1 and FP3 this weekend.

The second Qatar race weekend starts on Friday.