Dempster's barely pitched in the playoffs and looked eminently hittable in Game 1. Your other options were Morales, Uehara for six outs, or Tazawa for as many outs as you dare. Given that you used Tazawa for several high-leverage, high-stress outs last night, not to mention an insanely high-leverage out in the seventh, and you have Game 5 looming tomorrow, I can understand Farrell opting to give him a breather in the hopes that he'd be ready for a full inning tomorrow. So you're down to Morales or Uehara against Adams, Molina and Jay.
I can see going to Morales, with the idea that he can get Adams and Jay and hope for the best against Molina. But he's been so far from reliable in the postseason, I can completely understand Farrell avoiding him there. So now you're down to Uehara for six outs, which is at least 20 or so pitches, with Game 5 tomorrow.
It was a weird decision given that Lackey's relief experience is slim and none, but Rembrat's right; with Breslow going into the tank, the reliable arms in the bullpen are Uehara, Tazawa and Doubront, and that's it.
Maybe a question for another thread, but if this is the max of what can be expected from Buchholz/the Sox don't think Buchholz can pitch any more this season, should the Sox remove him from the roster and bring in Thornton or Britton? Neither of those guys is a difference maker, but they can hopefully at least get some outs in the middle innings if needed and maybe replace Breslow as a late LHP option.