Milan did not take up their option to buy Aster Vranckx after a loan spell from Wolfsburg, but the midfielder is reportedly returning to Serie A for Fiorentina, which could mean a Sofyan Amrabat exit.
According to Sky Sport Italia, the deal is practically done at a cost of circa €7m.
The 20-year-old Belgium international had also been heavily linked with a move to PSV Eindhoven, but the Viola dived in with a convincing proposal.
Milan paid €2m last season to have Vranckx on loan, but did not take up their option to buy after barely using him.
He had just 10 competitive appearances for the club, contributing one assist, and spending all of 318 minutes on the field.
Vranckx has two senior caps for Belgium, although he will not turn 21 until October.
The arrival of Vranckx at Fiorentina could well be a sign that they expect Manchester United to finally come in with an offer for Amrabat.
The Morocco international has been waiting all summer for the Old Trafford side to make a formal bid, which has not yet happened.
There is also interest from Liverpool and Atletico Madrid.