Solis had the talent and skill, but no will or discipline to be top professional athlete in the new circumstances where you work out when you want to work out compared to working out under "strong encouragement". He just wanted to have another big mac under his belly as quick as possible and hurt his knee thinking that... Vitali never hit him - it shows in his reaction.
Solis fought up to 270lbs once. Looked like the human blob fighting monte Barret. Didnt go down with bad knees then, even when he was 20 pounds heavier.