Lucas Torreira is facing as lengthy spell on the sidelines after fracturing his ankle at Fratton Park.
Arsenal have announced Lucas Torreira suffered a fractured right ankle during the FA Cup fifth round win at Portsmouth on Monday.
The Uruguay midfielder was carried off on a stretcher following a heavy tackle from James Bolton.
Torreira left Fratton Park on crutches wearing a protective boot after the Gunners moved into the quarter-finals with a 2-0 victory over the League One promotion contenders.
On Thursday, the Premier League club confirmed the 24-year-old sustained a break and a recovery plan will be determined following "specialist reviews".
Bolton's tackle was deemed to be fair by referee Mike Dean and the defender had no regrets over the incident, although he gave his best wishes to Torreira.
"It was a great tackle," Bolton told the BBC after the game.
"I timed it really well but it’s never nice to see someone down. I hope he's okay."
Arsenal, who will face Sheffield United at Bramall Lane in round six, also revealed that full-back Cedric Soares (knee) is aiming to be back in full training next week and Kieran Tierney (dislocated shoulder) has been able to train fully.
Sead Kolasinac (shoulder) hopes to be back in training by the of this month, while Calum Chambers (knee) is a long-term absentee.
Mikel Arteta's side host struggling West Ham in a London derby on Saturday.