Arsenal manager Arsene Wenger said midfielder Henrikh Mkhitaryan will miss Sunday's Premier League match against Southampton with a knee injury.
The 29-year-old Armenia international limped off after 61 minutes during Arsenal's 4-1 home win over CSKA Moscow in their Europa League quarter-final first leg on Thursday.
"Mkhitaryan has a knee problem," Wenger told reporters. "We don't know how bad it is. He will certainly be out for Sunday."
Wenger also said goalkeeper David Ospina, who missed the CSKA match, had suffered an ankle sprain in training and would be sidelined for up to three weeks.
Petr Cech started in place of Ospina, who has been Wenger's preferred choice for European and domestic cup competitions this season.
"He just moved back on a shot and sprained his ankle. Alone, without any contact. Just a bad movement," the manager added.