The England international ended a ten-year association with United when he left for the Italian side – where he spent last season on loan – in a permanent deal on deadline day.
Smalling signed for United from Fulham in the summer of 2010 and went on to win two Premier League titles as well as the FA Cup, League Cup and Europa League once each during his time at Old Trafford.
In a tweet, Smalling wrote: “It all began in July 2010, and now 10 years, 323 appearances, 2 titles, and 6 cups later it has come to an end.
“Utd is a special place, and we achieved special things together, something I am beyond proud about. An unrivalled culture, where winning is not a want it is a need.
“I leave not only a better player, but a better person, with Utd forever in my DNA. For that I have to thank all the staff, everyone connected to the club, and you the fans.
“Nothing surer you will continue to greatness. CS”