The seven-time MotoGP world champion's tough 2012 season got worse still at Silverstone, where he could only qualify 10th - 2.1s off the pace and over a second adrift of team-mate Nicky Hayden - and finish a lonely ninth.
"A very difficult weekend for us," Rossi told the official MotoGP website.
"In the dry we were always very slow. Except for the first practice in the wet, the rest was a disaster."
Rossi had an exhaust issue in the race, but said there was much more to his disappointing performance than a technical problem.
"After three laps I'd broken a pipe so I also lost some performance, but the problem is not that," he said.
"The problem is that we don't fix our problems and Silverstone remains a bad track for me, like last year. We have to work, we have to improve our pace and hope that in Assen we can be better."
Meanwhile, Alvaro Bautista and the Gresini team said it was more important to prove they could run with the frontrunners again in the British Grand Prix than to take risks trying to get a podium.
So far this year Gresini and new rider Bautista have struggled to recapture the form shown by the Spaniard's predecessor, the late Marco Simoncelli, in 2011.
But at Silverstone Bautista took advantage of changing weather conditions to claim his maiden MotoGP pole, and then stayed with the lead group in the race and finished just five seconds behind winner Jorge Lorenzo in fourth.
"It has been a great weekend and it has confirmed the progress we have made since Catalunya," said Bautista.
"I am so happy to finish so close to the podium.
"Maybe I could have attacked for third at the end but it would have been risky and I just wanted to run with them to the end and not make mistakes.
"We have brought home some big points for the championship which was important for me and the team.
"On pole and just five seconds off the win - that is a very positive weekend and I hope we can continue from here.
"Our objective now is to take another step forward and run with the front guys on a more consistent basis."
Team boss Fausto Gresini praised Bautista for aiming for a sensible finish rather than trying any risky moves on the leaders.
"It was wonderful to see Alvaro fighting at the front throughout the race," he said.
"It was the first time he has battled with these guys, he was careful not to make mistakes and he did a great job.
"Running at the front of this pack will also give us great data."