It's tough for this season.
Beckett has been performing better than Lester. Sure, he has that rough "Dice-K" inning but he still is giving us quality starts give or take. Lester has been a relative disaster. I'd like to see Beckett go at the deadline but only if he is replaced with an "ace" or at least a #2 SP. But, Lester has been so horrible I can't see the Red Sox doing s*** past September w/Lester on the mound THIS season.
Keep Lester, Trade Beckett, Look for playoffs NEXT year. That's what it feels like is the best option due to the fact that no quality SP can be had w/o sacrificing the farm probz till the offseason.