Mikel Arteta said he fears Riccardo Calafiori could join Arsenal's mounting injury list but vowed the Gunners "will be flying" against Liverpool on Sunday.
Calafiori limped out of Tuesday's 1-0 Champions League win over Shakhtar Donetsk at Emirates Stadium 18 minutes from the end after falling awkwardly on his right leg.
The Italian defender, a £42 million summer signing from Bologna, will be assessed in the coming days but Arteta admitted the initial signs were not promising.
"With Riccy he had to come off because he felt something," said Arteta. "I don't know the extent of it so in that sense not great news. He could not continue playing so, a bit of a worry."
Arteta also said it was "unlikely" Bukayo Saka would be available to face Premier League leaders Liverpool this weekend as he battles a hamstring problem while Martin Ødegaard (ankle) and Jurriën Timber (muscular issue) are also major doubts.
With William Saliba suspended following his red card at Bournemouth, Arteta could resort to using midfielder Thomas Partey at right back in a reshuffled defence after a subdued display against Shakhtar in which goalkeeper Dmytro Riznyk scored an unfortunate 29th-minute own goal to settle the game.
"I was expecting a bit of a drop off," said Arteta. "And the fact that we put so much in the first half and didn't get the distance in terms of the scoreline, I think we paid a bit for that.
"Then we made life more difficult for us because we started give the ball away in not the right areas and then it was more open. That didn't help. But we have now four days. Believe me, on Sunday we'll be flying.
"We'll review, have a day off, then two days to prepare the game. Sunday is far and we are more than comfortable that we'll be in the right place."
Ben White was substituted at half-time but Arteta said that decision was due to a desire to avoid a fourth red card for his team this season after the defender picked up a 34th-minute yellow card.
"Ben, it was my decision to take him out," added Arteta. "He had a yellow card and we've played enough with 10 men in recent periods. They had a lot of density on that side, a lot of players on that side, so I didn't want to take any chances there."