The Englishman has wanted to sign for the Italian club all summer, where he spent the past season on loan.
Now he is set to get his wish. According to Fabrizio Romano, talks between the two clubs are progressing quickly, and a deal should be thrashed about before the transfer deadline.
The 30-year-old will play in Serie A for the next few years, while United get some cash for their own incoming moves.
Smalling has not played for United this season, and has been training alone.