The toughest weekend of Valentino Rossi's MotoGP career, at least in terms of results, came to a close with 16th place in Sunday's Doha Grand Prix.

After the excitement of qualifying fourth for his Petronas Yamaha debut the previous week, Rossi approached the second Qatar weekend looking for solutions to rear tyre-wear issues that left him twelfth in the opening race.

Instead, the Italian only made one appearance in the top 13 (ninth in FP3) and was ranked 21st in both final practice and qualifying, his worst ever premier-class grid position.

The race brought little reward, The Doctor briefly gaining three positions at the first corner before settling back into 21st for the opening 8 laps.

Two retirements ahead plus passes on Danilo Petrucci, Takaaki Nakagami and younger brother Luca Marini eventually put Rossi 16th at the flag, 14.2s from Factory Yamaha race winner Fabio Quartararo and 5s from KTM's Miguel Oliveira and the final point.

Rossi's fastest lap was again only ranked 21st best, albeit within 0.9s of the quickest of the race by Ducati's Francesco Bagnaia.

"For sure it's not a great result and starting from behind is very difficult because at the end my pace was not a disaster, but it was not enough to stay with the first group, especially in the first laps," Rossi said.

"I lose a little bit too much because with the new tyre I was not strong enough. And after we are not very far, I mean everybody is very fast, in a line, all the riders and bikes are strong.

"I expect a little bit better, because this morning we find something not so bad that we improve. So I thought I had enough pace to stay with the group in front of me but at the end was not enough and I also didn’t take any points and it was a difficult race.

"We need to improve the rear grip with the soft tyres, because we all raced with the soft tyres and I suffered a bit. I was faster compared to last week, but it was not enough."

Rossi's race time was 1.2s quicker than the week before, but the race-winning time was almost 5s quicker.

The 42-year-old had been more competitive on hard tyres in the afternoon heat than on the soft rubber required for the night-time qualifying and race.

One of the changes made for the second weekend was a switch back to the aluminium swingarm rather than carbon fibre.

"We used the aluminium swingarm because I suffered a lot of rear vibration, after some laps the tyre suffers too much and starts to vibrate a lot. So with the aluminium it's a bit better, I have anyway the vibration, but it's more controllable," Rossi explained.

"Today we found some different settings also for the electronics that mean I can stay with a more constant pace. Unfortunately, we find just this morning and my position in the start was very bad.

"Now it's difficult because all the riders and all the bikes are strong, within ten seconds you are 15th! We have to see at the other race tracks, starting from Portimao, to understand if we are more competitive."

Excessive rear tyre consumption has been a frequent problem for Rossi in previous seasons, has Yamaha's switch to a 2019-style chassis for 2021 perhaps made the problem worse?

"No, I don’t think it's the chassis, it's not a big difference and the new chassis helps a little bit to turn but it's not enough," Rossi replied.

Qatar has traditionally been a strong venue for Rossi and the 2021 results are his worst-ever finishes at the circuit, but he's not drawing any conclusions heading into the first European event in Portimao.

"Every year changes a lot and anyway in Europe there are a lot of good tracks for me, so we hope that when we come back to Europe we can be stronger," he said.

While Rossi and team-mate Franco Morbidelli (on the A-Spec bike) both had Qatar races to forget, the Factory team won both grands prix with Maverick Vinales and then Quartararo.

The young Frenchman had suffered similar tyre problems to Rossi in the first Losail race but was able to fight through from ninth to first on Sunday.

"I think Quartararo did a fantastic race," Rossi said. "This time it looked like Maverick was struggling a bit like Quartararo last week and Quartararo is fast like Maverick this week! But why, sincerely I don’t know!"

Meanwhile, Petronas were left with a 12th place in each Qatar race, by Rossi and then Morbidelli.

"To be honest, I am quite happy that we have now finished these first two MotoGP races at Qatar, as we can now focus on analysing what has happened and be prepared for Portimão," said team principal Razlan Razali.

"It is what it is and hopefully we can do better there, especially as it was quite a good race for us last year with the podium of Franco."