Valentino Rossi says his shoulder problem is the worst injury he has sustained during his career, and the Italian claims he is riding at 70% of his potential.
Rossi has been carrying the injury since he crashed during a motocross training session in April.
The Italian says he still needs a lot of time to recover and admitted he will have to undergo surgery when the season ends if things don't improve.
"These things heal in three and a half months: I'm fully assisted, but only one and a half months have gone by," Rossi was quoted as saying by Gazzetta dello Sport.
"The shoulder bone tried to dislocate and broke the cartilage around it. I'm disheartened: this is the worst injury in my career.
"It will stay with me until the summer break, and in the meantime I will try to strengthen the muscles in order to support the shoulder. However, if I don't manage to, I will have to undergo surgery at the end of the season, and that will waste some months for me.
"I feel at 65-70% of my physical potential, but to race here, with these incredible spectators, will perhaps give me an extra push to do better than I could. We'll try to make he bike easier to ride in order to suffer less, and the rest will come from the heart."