Ben White was helped off the field during Arsenal's 1-0 win at West Ham on Sunday after suffering what appeared to be a knee injury, and manager Mikel Arteta offered little optimism when speaking to reporters afterward.
"We don't know but it doesn't look good at all," Arteta said. "He needs some further testing tomorrow probably and then we will know."
White was injured in a tackle on Crysencio Summerville in the first half. He was visibly in pain as a member of Arsenal's medical staff helped him to the sideline, and was later seen leaving the ground wearing a knee brace.
The timing could not be worse for the 28-year-old right-back. England manager Thomas Tuchel handed White a surprise recall to the national squad in March, and White was building a case for a spot in Tuchel's World Cup squad. Tuchel is scheduled to name that squad on Friday, May 22.
Arteta also revealed that Italian defender Riccardo Calafiori picked up an issue during the same match and was substituted at halftime. Arteta twice reorganized his backline in response, first moving Declan Rice to the right side of the defense before bringing on Cristhian Mosquera after Calafiori's withdrawal.
The potential loss of both players adds pressure to an already stretched Arsenal squad. Jurrien Timber, who can play on either side of the back four, is also currently unavailable. Arsenal have two Premier League games remaining and face Paris Saint-Germain in the Champions League final on May 30.
