Yes, Rafael Leão has had, shall we say, some issues with the Milan management, most likely linked to his contract renewal and, above all, the financial terms.
He’s right to leave; he’s already given so much to Milan and could have given more, but given the problems he’s encountered with the various managers who’ve come in since Stefano Pioli, he hasn’t had any consistency.
At Galatasaray, he’ll form a very promising strike partnership.
Who do you think Milan will bring to replace Leao? Amorim has been using the 3-4-2-1 formation in recent friendlies though. Maybe he isn't thinking of playing with wingers?
But he would still need a backup plan if this strategy doesn't work out. That's why I think they will sign a new left winger. They can handle it for a reasonable price if they manage to sell Leao for around €50m.

I haven’t the faintest idea who might take Rafael Leão’s place; certainly, if he is sold, the club’s management will have to make a major signing to fill the gap left by the Portuguese player.
Amorim has used a formation that Milan are not very used to, but we’ll only find out at the start of the season whether the 3-4-2-1 will be retained or whether he’ll revert to fielding wingers.
It remains to be seen who will play in the attacking midfield role behind the striker.