Unfortunately I don't see Beckett and Lester turning it around. I would bet that at best Beckett remains inconsistent for the remainder of the season while missing at least one and possibly two turns in the rotation.
Lester has so many mechanical issues plaguing him that even talking about him turning it around mid season this season is simply not realistic. It is so bad at this point that if he comes out again and cannot even find parts of the plate while not even being able to get squared up, they may have to shut him down. If he actually throws again like he threw in the last start he could actually hurt himself and I am not trying to be facetious.
If that happens, they need another starter and guys that are really supposed to be back of the rotation guys are going to have to step up like they are front of the rotation guys.
If the season ended today and the Sox had to win one game, Buch would in my view be the choice. If they got in to a short series, they would be stuck with Beckett and pray for rain.