In the beginning of the season, probably nothing. I can't claim that i've watched many of Torino's games in the Serie A but i don't think that there's something that suggests Darmian will be a regular starter from day one. To me he seems more or less like a utility player. He can play on both flanks, in fact he played as a (defensive) left wing back more than anything else at Torino, plus his height and strength allow him to be a choice in the center half position too.
Probably a better defender than Valencia, tactically inept (as any Italian footballer who gets called in the NT) with and without the ball which is something that LvG rates highly but not as pacey as Valencia and with zero experience at a top club which doesn't defend with 8 or more players behind the ball. We'll have to wait and see if he's going to prove himself significantly better than Valencia on the pitch.
Right now, he seems like a rather cheap but clever signing who will start his United career as a back up for both Valencia and Shaw. Until we get the chance to watch him on the pitch the only "safe" conclusion we can draw is LvG's intention to have enough options at both FBs positions so he won't have to Smalling, Jones or even Rojo and Blind there.
