Odds are Coyle will be added to the 40-man this off-season. However I don't think he's ever going to be viewed as a full-time option for the Red Sox. He could certainly be dealt away this fall/winter, but his injury history (as Rip noted), his positional limitations, and his dearth of upper-level minor league experience will combine to reduce his value.
He's obviously blocked here at his natural position, and has only played 18 G at 3B this year (his first pro experience at the position). I don't want to jump to any small-sample conclusions, and I certainly don't place a whole lot of faith in fielding statistics (particularly the more archaic ones based on chances & errors, like his paltry 1.78 RF/G in 38 TC w. 6 E). But to expand on Joe Dokes' comment, I don't think it's a stretch to assume his 5'8" build is less than ideal for the hot corner in terms of both lateral and vertical range. (In a somewhat dated piece at Fangraphs,
Dave Cameron agrees; FWIW, that post was a follow up to
this one, and a precursor to
this one, all related to height considerations at 3B vs 2B).
Second basemen are typically positioned 20-30 feet further away from home plate than 3B are, and balls hit to second are usually softer hit, so 2Bmen can take more steps to get to where the ball is hit and they have more time to do so once they react. On slow rollers and routine grounders, I'd have no doubts about Coyle's abilities playing third in the majors. But on dives to his left or right and on leaps for liners over his head -- especially when playing in with runners on and less than 2 outs -- he's at a distinct disadvantage. For that reason, not only do I not see a future for Coyle at 3B, I don't view him as much of a utility option in the majors either. So it's really full-time 2B or bust.
If the club holds onto him, he projects to play 2B almost exclusively in Pawtucket with Cecchini (unless he's traded) doing the same at 3B. I just can't buy into any Cecchini in LF/Coyle at 3B theories, and it goes well beyond the logjam of outfielders. If Coyle handles AAA pitching well, he's a solid trade chip 14-18 months from now for a team with a 2B opening, which limits the market since 2B is where many failed but good-hitting SS and 3B end up. If he's only mediocre at the plate, he's nothing but a throw-in.