Arsenal defender Kieran Tierney faces a spell on the sidelines after dislocating his right shoulder in Monday's Premier League win at West Ham United, the club said on Wednesday.
The 22-year-old left back was substituted just before the half-hour mark in Arsenal's 3-1 derby victory at the London Stadium after falling awkwardly and used his shirt as a makeshift sling as he left the pitch.
The club said Tierney would have another assessment while British media reported that the Scotland international could be out of action for three months if he has surgery.
Arsenal, who won their first match in all competitions since late October, visit Belgian side Standard Liege in their final Europa League Group F match on Thursday and Tierney is among a number of senior players who will not make the trip.
The Gunners will qualify for the last 32 knockout stage with a win or draw or if Eintracht Frankfurt lose at home to Vitoria.
The club said Granit Xhaka suffered a concussion when he was struck in the head by the ball from close range and the Swiss midfielder will miss the Europa League match and Sunday's Premier League clash at home to Manchester City.
Record signing Nicolas Pepe will miss the Liege game as well after he bruised his knee, while Hector Bellerin has a tight hamstring, with both players to be assessed before City's visit.
Dani Ceballos (hamstring) and Rob Holding (knee) remain sidelined but are set to return later this month.