Decision-making under uncertainty. Farrell is not good at it.
The counter argument is the "manager of men" thing here though. Buchholz probably wants to be a starter, had been told he was getting another chance to do so, and Farrell felt yanking the rug out from under him was a bad idea longer term. But of course, the season is nearing short term, if not there already.
The bottom line is that they've got a terrible bullpen. And that's mostly on Ben Cherington's decision to give Koji 2X $9 instead of giving Miller 4X $10 to beat the Yankmes, and DD's bad luck in this years trades, and the failure to develop anyone except Barnes as a hard throwing bullpen option, and perhaps some on Farrell's erratic usage patterns, slow hooks, and failure to exploit matchup advantages.
Kimbrel, Koji, Tazawa, Carson Smith -- Barnes, Ross, Layne, Hembree
Kimbrel, Koji, Tazawa, Zeigler -- Barnes, Ross, Layne, Hembree
Kimbrel, Koji, Tazawa, Zeigler -- Barnes, Ross, Abad, Hembree, Buchholz
Those aren't terrible bullpens. Sure, there have been injuries, and DD has reloaded with Zeigler and Abad, both of whom were really good until they got here.
I think the most emblematic pitcher for me in terms of Farrell's ability to make the whole less than the sum of its parts is Noe Ramirez. 11 games of seeing whether or not he could function in a Carson Smith role. Sox go 2-9. (Not saying Noe was responsible for all those losses.)