Arsenal manager Arsene Wenger has revealed Mathieu Debuchy faces a six-week spell on the sidelines with a hamstring injury.
The right-back made his first appearance of the season in the 3-1 win over Bournemouth last week, but was forced off after just 16 minutes with a muscular problem.
Debuchy underwent a series of tests to determine the extent of the issue and Wenger has now shed light on the situation.
"Debuchy has a severe hamstring injury, he could be out for six weeks," Wenger said at a media conference ahead of Saturday's trip to West Ham.
"It is frustrating for me, but even more for him. He has an exceptional attitude and works very hard.
"He is very disappointed. But he is a fighter and will come back."
Wenger was initially hopeful Olivier Giroud would be ready to take on West Ham after missing Arsenal's EFL Cup defeat to Southampton in midweek, but the striker now appears set to miss this weekend's encounter too.
"I thought Giroud would be available, but the last news I got is that he might not be available," Wenger added.
"He will be tested Friday. But he will be available for [Basel on] Tuesday."
Danny Welbeck, meanwhile, has returned to training after a lengthy spell on the sidelines with a knee injury, although Wenger has warned the England international is still some way off making his comeback.
"It is great to have Welbeck back in training, but he is at least four weeks away from returning," the Frenchman concluded.