The striker, 29, has been in hospital since being taken ill on the team's plane following Milan's 3-2 win over Roma in Serie A on Saturday.
The club said the procedure to close a small opening in the player's heart was carried out by Doctor Mario Carminati.
Milan physician Rodolfo Tavana was also present.Continue reading the main story Bari (1999-2001) 48 league games, six goalsRoma (2001-06) 118 games, 39 goalsReal Madrid (2006-08) 19 games, two goalsSampdoria (loan 2007-08) 22 games, 10 goalsSampdoria (2008-11) 96 games, 35 goalsAC Milan (2011-) 26 games, eight goals
Coach Massimiliano Allegri said his team were "playing for Cassano" in the Champions League on Tuesday.
The Italian giants drew 1-1 with BATE Borisov in Belarussia and lie second to Barcelona in Group H.
Before the game, Allegri said: "We will be playing for Antonio. Quite apart from the sporting element, what counts most is the human aspect and we're all feeling sad.
"Antonio was doing well and he was in good shape and I hope he'll be back as soon as possible.
"I can't diagnose him but he sounds fine. It was completely unexpected and has nothing to do with football. Let's leave it to the doctors and I think everything will be fine."
Cassano had reportedly developed problems with vision, speech and movement on the plane and had tests on his heart and brain in hospital.