Considering his fitness issue and his play style at the same time I think Allegri should consider him as DM (rotation for Marchisio). The guy is always calm, keeps it simple and has decent passing. He's reliable too. He doesn't give enough as B2B IMO, mainly due to lack of stamina.
Until the end of the season we should try Lemina as B2B. If he fails then we need another player who would be capable of starting. I think Lemina would do just fine, and Marchisio would have a reliable player to rotate with. I don't trust Hernanes at all tbh, shoule be sold to make a room for a proper AM.