Arsenal teenager Krystian Bielik was injured playing for the under-21 side in a defeat to Reading.
The mid-season signing from Legia Warsaw was withdrawn after only 54 minutes of the clash, reported the club's official website, appearing to have a thigh injury.
He was playing once again in central defence, for the second week in a row, with Jack Wilshere and Abou Diaby occupying central midfield spots.
Bielik was booked in the first half, and while the extent of his injury is not yet known, Arsenal are unlikely to rush the 17-year-old back before he is ready.
Any missed game time would be a blow for the imposing Pole, who has only just broken into the under-21 side over the past month.
Arsenal lost the game 1-0 through a goal from Niall Keown, son of former Invincibles defender Martin, with their senior playes unable to turn the game in their favour.
Hat-trick hero of a week ago against Stoke City, Alex Iwobi was up front with Ainsley Maitland-Niles and Serge Gnabry preferred, and Daniel Crowley completing the midfield.