Arsenal manager Mikel Arteta says the injury suffered by defender Riccardo Calafiori during the 1-0 win against Shakhtar Donetsk is "a bit of a worry".
The Italian, who joined from Bologna in a summer deal worth up to £42m, went off in the second half of Tuesday's Champions League game after seemingly twisting his right knee.
Arsenal are already without key attacking players Martin Odegaard and Bukayo Saka, while defender Jurrien Timber is also injured.
"He felt something, he could not continue playing," Arteta said of Calafiori.