Gattuso due back in six months
The Italy World Cup winner, badly missed by Milan in Sunday's 4-2 Serie A defeat at Juventus, tore ligaments in the 1-0 win over Catania earlier this month.
"The cruciate ligaments in the right knee have been reconstructed. The surgery was perfectly successful," Milan said in a statement.
"The recovery time is estimated at around six months."
The operation in Antwerp, overseen by top surgeon Marc Martens, had been due to take place on Friday but was brought forward.
Milan, joint-third in Serie A and nine points behind leaders Inter Milan, have ruled out buying a replacement for the 30-year-old in January when midfielder David Beckham will join on a short-term loan from Los Angeles Galaxy.
Italy, who top World Cup qualifying Group Eight, will be without Gattuso for February's friendly with Brazil in London, March's qualifying trip to Montenegro and the visit of Ireland in April.