Mikel Arteta was waiting to learn the extent of an apparent thigh injury suffered by Bukayo Saka, who limped off during Arsenal's 3-0 win over Sheffield United.
Saka helped set up Arsenal's opener, the first of two goals for Alexandre Lacazette, as they cruised to victory over the Premier League's bottom club.
But the Gunners had cause for concern when rising star Saka came off in evident discomfort, having not been fouled, four days before the second leg of a Europa League quarter-final with Slavia Prague that is all square at 1-1.
Still 10 points adrift of fourth-placed West Ham in the Premier League, Arsenal's Champions League hopes for next season rest on their Europa League campaign and Arteta knows the Gunners can ill-afford any absence for Saka, who played two key passes at Bramall Lane.
Arteta told BT Sport of the injury: "He had some discomfort in his thigh. I don't know what it is. The last thing we need is another attacking midfielder with another issue."
Speaking in his post-match media conference, Arteta added: "I don't know the extent. For Thursday, David [Luiz] and Kieran [Tierney] will be out.
"Emile [Smith-Rowe] and Martin [Odegaard] we don't know yet. Let's see how Bukayo develops in the next few days."
Gabriel Martinelli scored Arsenal's second goal, his first of the season having had his development interrupted by injury issues of his own.
But Arteta is confident Martinelli, 19, is progressing in the right manner.
"I'm asking you to be patient because we have a lot of 19 and 20-year-old players," said Arteta.
"It's not common and Gabi is on the right path, with the right development. He's got an incredible attitude and talent."