Arsenal boss Mikel Arteta says Kai Havertz's "unexpected" season-ending injury came when the striker blocked a shot during training.
The Germany forward will miss the remainder of the campaign after sustaining a hamstring injury during a winter training break in Dubai.
Havertz will have surgery on the issue and is not expected to return until pre-season.
"He was going to stop a shot after a set-piece and stretched the leg and he felt his hamstring," Arteta said.
"We were having a great camp in Dubai, recharging, training, having some time off and connecting again with nature and a different environment.