I am not downplaying it, but a guy who has reached Lester's level of sustained success at the MLB level got there by working out all the kinks over several seasons. He maintained his success by repeating his delivery thousands of times. He has built up muscle memory that is robotic like every other successful ballplayer. I don't think that he could have strayed from his mechanics so radically that he would have to rebuild the whole thing. Usually, after achieving his level of sustained success, it is a minor fine tuning situation as opposed to a major overhaul. He should have worked it out over the course of a season. I think there is very possibly something else at work whether it be mental or damage to his arm.
People think that he will regain his old stuff if he abandons the cutter. I don't think it works that way. Dick Radatz used to tell an interesting story about adding a pitch to his arsenal. He was at the height of his success when one Spring Teddy Ballgame suggested that he had the perfect motion for a sinker. Teddy Ballgame was like a walking god at Spring camp. If he talked to you, you listened. Radatz worked on the sinker all spring and it was coming along really well, but when they broke camp and Radatz reached for his bread and butter, the big heat wasn't the same. He had lost a foot off his fastball and he never regained it. He had no discernable arm injury. I don't believe that he ever had arm trouble, but his dominating gas was gone. He went from being an intimidating overpowering force to a punching bag seemingly over night.
I am sure someone will research this and my recollection could be faulty on Radatz. Maybe William s suggested a slider and not a sinker, and he may have hit the DL at some point, but the main point stands. Lester's 4 seamer is not what it had been. Others have inferred that the cutter could have a negative impact on other pitchers. I am just saying that abandoning the cutter may not bring back the fastball.