Milan goalkeeper Mike Maignan will be out for 10 weeks following surgery on his left wrist.
The Serie A giants signed Maignan from Lille in May for a reported €15million fee to replace Gianluigi Donnarumma, who departed for Paris Saint-Germain on a free transfer.
The France international has started the season in fine form, starting every game as Stefano Pioli's side collected 19 points to match their best start in their opening seven Serie A games.
However, the 26-year-old suffered a wrist injury against Liverpool on September 15, with the problem worsening 13 days later in a clash with Atletico Madrid's Luis Suarez.
Milan announced on Wednesday that Maignan had undergone surgery, adding that he will rest for six weeks before beginning the rehabilitation process. He is expected to return near the end of December at the earliest.
His potential 10-week absence means he could miss the entirety of the remaining Champions League group-stage fixtures, alongside crucial domestic clashes against fellow title hopefuls Roma, neighbours Inter, and Napoli.
Up until his injury, only Bartlomiej Dragowski (1.62) boasted a better goal prevention figure than Maignan (0.94) among Serie A goalkeepers.
The news prompted Milan to sign 38-year-old goalkeeper Antonio Mirante on a free transfer until the end of the season, with Ciprian Tatarusanu Pioli's only other option until January.
Meanwhile, Milan defender Theo Hernandez has tested positive for COVID-19 after reporting back from international duty with France, meaning Milan will be without the left-back for the visit of Hellas Verona on Saturday.