It's not just that the pitching fell apart, the pitching was completely injured and there was no rotation to rely on in September. They needed to drag an ancient Wakefield out there (6.30 ERA last 4 starts of his career). Kyle Weiland had 3 horrible starts (9.26 ERA). They traded for Erik Bedard and it didn't work out (3 starts 5.25 ERA, didn't get out of the 4th inning the last two starts). They had Andrew Miller for two starts (15.63 ERA) before he was sent off to the pen. Then the C&B crew (Lackey 5 starts/9.13 ERA/4.17 FIP, Beckett 4 starts/5.48 ERA/4.63 FIP, Lester 6 starts/5.40 ERA/3.75 FIP) all pitched poorly at times and got lit up in the last few weeks, but may have been a little bit of bad luck.
In Septemeber, the Sox were 7th in offensive fWAR and 6th in wRC+. They were 28th in starting pitching fWAR (30th in ERA, 29th in xFIP, 29th in innings thrown) and 15th in reliever fWAR (lead MLB in innings thrown).