Jorge Lorenzo conceded he struggled to cope with the physical demands of MotoGP during Friday's practice session at Silverstone following an absence.
Lorenzo had missed the past four rounds after fracturing vertebrae in his back in a crash during practice at Assen in June.
The Spaniard targeted the British Grand Prix as his comeback race, but he struggled in practice, finishing last on the combined times and 3.682 seconds off leader Fabio Quartararo, and Lorenzo acknowledges he needs to be patient.
"It has been a tough day because you never feel very comfortable when you are in the last positions and you are last like I was," Lorenzo said. "It is the situation that we have now and to try to push creates crashes and the consequences that you can imagine, so for the moment it is the situation that I have.
"My back has not healed completely, and I have pain, especially after FP2, and I have lost muscle mass and my physical conditions is not used to MotoGP because of two months without bikes. Little by little, I need to be patient to get through this race."
Lorenzo added he is unsure if he will be able to complete the full 20 laps on Sunday.
"I don’t know yet. After the second practice I started to feel more pain on my back compared to yesterday," the Repsol Honda rider said. "Every practice that I will do, it will be worse for my back, but I need to manage it and try to manage the right laps, to be able to make the maximum pace and get the right laps without getting worse with my injury.
"Almost everywhere I have problems physically, in braking, also in the middle of the corners with my neck as my neck lost a lot of muscle, and the change of direction.
"I have the pain in the back and I move very slowly, so everything I do very slowly. It shows that in my three-and-a-half seconds off to the fastest ones."