You can't expect him to impress against Celta away with a full rotation of the whole team. He obviously still has to adapt to Barca and la liga in general, so starting this game after weeks without a game is very difficult and i think he did ok.
He should've started some easier home games, with a better midfield, and being subbed in for some games we were leading. That's how you show a new/young player you trust him and not by letting him watch from bench/stands for weeks and then suddenly throw him in an away game we often lost in the last seasons.
He is still showing the expected weaknesses in positioning but getting games is the only way to really improve that and there were enough games where he could've played. Hope Valverde gives him some more chances this season.