He's 18 years old. Of course he's lacking something to break his way into the starting XI even against small teams. It's called experience, and probably maturity as well. He's young, not fully developed, nowhere near his physical peak. He's played a ton of minutes considering all that.
There is no u-18 player who play more minutes in serie a this season than coman.
And he came from other league.
Juve or allegri did not ignore him
Even 22 years old like morata must adapt to become starter at juve.