He was out with a very serious injury until November or so... and who knows how long it was before he was legitimately match fit and ready to play regularly. Also considering how much everyone tapped over Sandro's short-term return to form, Allegri would have been crucified if Spinazzola had performed poorly while he was still finding fitness and form. This is probably one of the more understandable cases of Allegri's babying new players.
So the choice was clearly MDS over Spinazzola and now here we are predictably relying on Spinazzola. I think he should have got into one or two of those matches minimum. It's just so poor and unnecessary to keep him out for a month. Even if Allegri thinks MDS is better for whatever reason, surely you work in the substitute somewhere.
