Honestly though I think the latter Sylvia became a better fighter and indeed picked up his best wins over Arlovski, he could be dull and fight safe but I think his boxing improved and he used his reach more effectively.
He was always vulnerable to fast aggressive opponents although I do suspect the back injury did play a big part vs Randy, one of the shadiest episodes in MMA history IMHO, Couture using his own gym to find out a fighter was injuried and then forced him into a match with the UFC threatening to strip him if he didnt and had an op instead.
Post Fedor though he clearly went off of a cliff, got paid a load of money, didnt have much else to aim for and came back about 30lbs heavier.