4:04pm: The Bucs plan to use Bell off the bench until the All-Star break, at which point he'll return to Indianapolis, Travis Sawchik tweets. This is disappointing, but perhaps not surprising, given the roster issues discussed below. I still expect the Pirates to do something before the trade deadline to clear space for Bell.
3:16pm: The Pirates have officially promoted Bell and have optioned Glasnow to Indianapolis, a possibility noted below. I wouldn't interpret Glasnow's departure as a comment on his role with the Pirates, perhaps not even in the short term -- with the All-Star break coming up, they didn't figure to need him for at least a week or so anyway, so optioning him allowed them to add an extra bat. I assume he'll be back at some point very soon.
The Bucs had already released tonight's lineup, and Bell isn't in it.
Bell's promotion is, in my opinion, slightly overdue. Bell has been terrific in Indianapolis, while John Jaso has faded as the season has progressed. Bell is currently batting an excellent .324/.407/.535, with a 13-homer total that already matches his career high. Offensively, he doesn't seem to have significant weaknesses, at least not any that Triple-A pitchers are finding -- he's always hit for average and plate discipline, and now his strong approach at the plate is helping him generate power as well. The knock on Bell since his transition to first base has been his defense, which is widely considered to be a work of progress. Since he's a very promising offensive prospect, though, the Pirates should probably be willing to deal with occasional rough patches.
Jaso, meanwhile, is headed in the wrong direction. He has a solid .352 on-base percentage, and his defense is steady. But he struggled in June (although he's hit well in a small sample thus far in July), and his ground-ball-heavy and weak-contact-heavy batted-ball profile suggests the Pirates aren't likely to get much more out of him. Bell is arguably a better player than Jaso right now, and clearly the one with the brighter future.
From a roster-construction perspective, though, the problem with replacing Jaso with Bell is that Jaso isn't valueless, thanks to the aforementioned OBP and defense, and the Bucs might have trouble fitting Jaso onto a crowded bench. They could potentially trade someone, but unless that's Jaso himself, it would be somewhat strange for them to subtract one of their very productive bench players after their recent stretch of good play. They could send Adam Frazier back to Indianapolis to clear space for Bell, but Frazier has been great, and such a move would leave the Pirates pretty thin on middle-infield help Of course, they could deal Jaso. And I hate to speculate about this, but it's possible to imagine scenarios where David Freese moves from the bench to become the Pirates' regular at third base, given Jung Ho Kang's situation. We'll see what unfolds. In the short term, as David Todd points out, the Pirates could easily just send Tyler Glasnow back to Indianapolis for a bit, since he won't pitch before the All-Star break anyway.
Anyway, Bell becomes the Pirates' latest top prospect to earn a promotion, joining Jameson Taillon and Glasnow. The roster also features fellow rookies Frazier and Chad Kuhl, giving the Bucs a distinctly young look for the first time in awhile. It's been an uneven Pirates season so far, but the huge group of promising young players earning promotions has been a highlight.