Arsenal have revealed midfielder Granit Xhaka has been ruled out for around three months after sustaining knee ligament damage against Tottenham last weekend.
The Switzerland international returned to Arsenal's starting line-up for the north London derby after serving a three-match ban but was forced off in the 82nd minute.
Xhaka was accidentally caught by Lucas Moura in the Gunners' 3-1 win at Emirates Stadium after the Brazil international had initially been challenged by Thomas Partey.
Speaking after Sunday's victory, Arsenal manager Mikel Arteta admitted he was "pretty worried" about a potential injury lay-off for the 29-year-old.
Arsenal have now confirmed that, while Xhaka does not require surgery, the injury is serious enough to potentially keep him out of action for the rest of 2021.
"Further to injuring his right knee during Sunday's match against Tottenham, assessments and scans have confirmed Granit Xhaka suffered a significant injury to his medial knee ligament," a club statement read on Wednesday.
"A specialist consultation took place in London on Tuesday evening, which has determined that Granit does not require surgery.
"Granit's recovery and rehabilitation programme will start immediately, and we are aiming for him to be back in action in approximately three months.
"Everyone at the club will now be supporting and working hard with Granit to get him back on the pitch as soon as possible."
Xhaka will therefore miss November's meetings with Liverpool and Manchester United and may be doubtful to face Manchester City on New Year's Day. Arsenal visit Spurs two weeks later.
He will also play no further part in Switzerland's World Cup qualifying group-stage campaign.
Despite only Bukayo Saka making more appearances (46) for Arsenal last season in all competitions, Xhaka was heavily tipped to join Roma during the most recent transfer window.
The former Borussia Monchengladbach man instead agreed a contract extension with Arsenal and started four of their opening six Premier League games this term.
Xhaka ranks second to Albert Sambi Lokonga for tackles won among Arsenal players in the Premier League in 2021-22 – seven compared to nine, having played a game fewer than his team-mate – while only Pierre-Emerick Aubameyang has won more duels (25) than Xhaka's 22.