Morata has never been an incredible scorer in his career. In terms of this aspect he is overrated. However he can still be supportive on attacks in an effective way. But I don't know playing as the centre-forward at Milan would be good for him. Pioli uses a 4-2-3-1 formation most of the time. I think Morata shouldn't be the only player on the front to be relied upon.
He can be more effective when he has a companion beside him. I wonder if Pioli has such plans to make a change. Because I don't remember him using a formation with two strikers. But if you have Morata in your team it is better to adopt such formation just as Atletico Madrid have done.
Morata was once the lone striker in the Spanish national team and that went well in my opinion, in terms of quality Morata is a player who is quite agile in his movements and takes up empty space.
I think Morata won't be a problem if he gets good support from the flanks or in the middle to score goals, Morata is the type of player who can be an executioner and can also be a good feeder for his teammates, for a striker he's not selfish and that's very good for the team.
If he plays as lone striker at Milan one role - duty combination comes to my mind which could still make things more positive for him though. He should play as a deep lying forward with support role. He should be a completely team player in his position to be really helpful for his team. If he becomes selfish like waiting the ball to come to himself and uses all the chances by himself only then he wouldn't do well there.
I assume Pioli would think the same as me considering Morata isn't a scorer who has high composure and who is great at finishing mostly. But this role would fit him great to score and make assists quite much as he has done at Atletico Madrid so far.