Tomori suffered a knee injury in the first half of the 3-1 Coppa Italia win over Genoa on Thursday which required a keyhole surgery to fix.
Milan have announced the defender is expected to be sidelined for around 30 days.
Tomori thanked the fans for their support, saying: “Successful surgery today, aiming to return to the pitch as soon as possible.
"Thank you all for your messages of support."
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