He has actually played an entire season in the Premier League at full back, when he was at Watford.
Ake is a strange case - he was a DM when he was at Chelsea (in the youth team, and in his very limited first team appearances), then a full back at Watford - and only started being played regularly at CB when at Bournemouth.
He was 21 by that stage, so had spent his entire development career to that point being played anywhere but CB.
Given how Pep asks his full backs to tuck into central positions when we're attacking up the pitch, Ake has all the attributes to fill that side of the role perfectly.
He definitely won't provide the attacking threat Mendy has in him, but against teams where we want a more solid defensive option, he'd actually be a very good choice at left back.