Could he be a major-league regular?

Probably not a good one. He’s established that he can dominate AA pitching, and he’s been a pretty good hitter in AAA over the last few years. As a regular, he’d probably be a .270 hitter with 10-15 home runs. He’s not a bad outfielder, but he’s not a real good outfielder either.

He’s another player who is almost certainly more valuable than the twelfth or thirteenth pitcher on a staff, but won’t get a chance to prove it. If you’re an American League team, and you carry twelve pitchers, you’ve basically got five spots for the bench/DH. Montanez probably isn’t good enough of a hitter to be any more than a spot DH. You need to have a backup middle infielder and a backup catcher. Montanez is competing for one of the two remaining spots. He can’t play center field or third base, so you have to decide if you can carry a corner outfielder/first baseman who’s NOT an outstanding hitter. Most teams will decide that they can’t.