Two things I've read about this:
1. From the dweeb a couple of days ago:
Perhaps more significant, however, is what Devers does defensively. He’s committed three errors (two fielding, one throwing) in five games at third base in Pawtucket, a reminder that he remains a work in progress at the position. It’s also worth noting the defensive transition to the big leagues at third base can be particularly daunting, given the need to stand the closest to home plate while dealing with bat speeds that are rarely replicated in the minors.
If it looks like he’ll need a while to field his position reliably, his timetable for the big leagues may be pushed back to September. If he shows rapid improvement, then he could accelerate his timetable, depending on what kind of trade the Red Sox make to address their worst-in-the-AL production at third base.
2. The second is a very valid point from PA about how important our left side defense is with our LH heavy rotation. That doesn't necessarily make the defense more difficult, but it does make it more important. Perhaps not the ideal situation for a player who is still a work in progress defensively.