Arsenal boss Arsene Wenger says Aaron Ramsey and Mikel Arteta will miss games against Galatasaray and Chelsea because of injury.
Both were substituted during Saturday's 1-1 draw with Tottenham, with Arteta, 32, suffering a calf problem and Ramsey, 23, a hamstring injury.
The Gunners play Galatasaray in the Champions League on Wednesday, before facing Chelsea in the Premier League four days later.
"They're out for Chelsea," said Wenger.
"It is a concern that Ramsey did his hamstring," added Wenger.
"I don't understand because he had a good rest this week and I didn't play him against Southampton on Tuesday.
"It's a surprise because he's usually a resistant boy."
Jack Wilshere, who took a knock to his ankle against Spurs and was substituted after 63 minutes, is doubtful for the visit of Galatasaray.
Wenger said he was concerned by the number of injuries Arsenal have - with striker Olivier Giroud (broken tibia) and Mathieu Debuchy (ankle) out for a lengthy spell and Theo Walcott still a few weeks away from returning to training after sustaining a cruciate knee ligament injury in January.
He added: "Suddenly we are short because we lost three players [against Tottenham], and let's not forgot that we already have two out long-term plus Theo Walcott, that's six.
"Abou Diaby is not ready, so it starts to become a worry."