He's a defender now. CB/LCB mostly. If he's a left back, then definitely a defensive one. Wingback role would expose him badly.
He has a lot of similarities to Danilo. Both need actual wingers ahead of them. AS hasn't had one on his side for a long time. Last time it was Mandzukic in Allegri's 4231, then nothing for a few years and now Chiesa plays of the left from time to time.
I'd like us to play a symmetric formation again, without leaving one band for a single player to cover. Thats' what happened to AS a few years ago.