This is a good deal for Milan. Despite Nkunku being quite talented, his performance with Chelsea is not as good as expected. He scored only 3 goals in the last season. Also, Nkunku's chances of getting a place in Chelsea's squad this season are quite low. Because there are talented players like Fofana, Sterling, Mudryk, Delap Gittens and Pedro. So Nkunku has already been dropped from Maresca's match plan. In this situation, it is better for Chelsea to make money by selling Nkunku. From what I heard, Milan management is willing to spend 36 million for Nkunku.
Maybe Nkuku wasn't in the right place and didn't give his all, don't you think? Let's say that on paper, Milan made a good move, and this small change could provide a boost they need right now; they haven't had a great start to the season.
I hope I'm not wrong, but I think Nkuku wasn't comfortable in some way, or at least wasn't giving his all. I think that if he can settle in well and find the right teammates, he could bring great results to Milan.