The 39-year-old scored twice before suffering the injury as the Serie A leaders recorded their first league win in Naples for more than 10 years.
A club spokesperson said it could take between two and three weeks for Ibrahimovic to recover.
The Swedish striker, who has scored 10 goals in six league games this season, will likely miss Europa League matches against the French side and Group H leaders Lille and Scotland’s Celtic on Thursday and on Dec. 3, respectively.
10 goals in the first 8 matches at 39: ladies & gentlemen, the Lord of Records 🔴⚫10 gol nelle prime 8 di A, a 39… https://t.co/sJkIKPTlQu
— AC Milan (@acmilan) 1606089143000
Ibrahimovic could also miss the final group game against Sparta Prague. Milan are second in Group H with two wins out of three matches.
The Serie A leaders have 20 points after eight league games and face 15th-placed Fiorentina on Sunday.