Don’t forget the Sox have already 1) added him to the 40 man roster 2) called him up to the bullpen before so that ship has actually already sailed, and 3) as I already said, it’s hardly an unblazed trail, with plenty of successful starting pitchers beginning their career as relievers.
There are some benefits. Shorter, more frequent outings let him bounce back from bad ones quickly, ample opportunities to work on secondary pitches against actual MLB hitters. There are several MLB teams that use the bullpen as a starting point for most of their best starting pitching prospects, notably the Dodgers. But what do they know?