Simple fact is that he just flat out got tired. Despite that 3 plus ERA, he's been nothing but lights out this year save for September. You add the already worked bullpen with the loss of Bedard and Beckett along with Buchholz's prolonged absence and the recent issues of Lackey and Miller and you're looking at a bullpen that's gonna crash.
If Beckett, Lester, Buch, and Bedard are able to contribute in the postseason, I still trust our bullpen to shut people down, hopefully they'll have had some rest come the start of October.