Danny Welbeck was taken straight to hospital after suffering an ankle injury during Arsenal's goalless UEFA Europa League draw with Sporting CP, coach Unai Emery has confirmed.
Welbeck returned to the starting line-up for the match at the Emirates Stadium and wasted an early chance to break the deadlock.
His second opportunity saw the England international head straight at Renan Ribeiro, but the striker landed badly on his ankle and left the field on a stretcher.
Emery confirmed the injury was "serious" but the Gunners will wait to learn the full extent of the 27-year-old's issue.
"The worst news tonight is these injuries," the Spaniard told a media conference. "We think [they are] serious injuries.
"He has been taken to hospital. We are going to wait [for updates].
"Danny was working well. He is a very good person, his commitment to us is very big.
"This injury is very big for him, for us and for everyone."
Arsenal also saw defender Stephan Lichtsteiner depart with an injury with 16 minutes remaining, and Emery hopes that is not as serious as Welbeck's problem.
"Lichtsteiner is muscular," he added. "We will know [more] about his injury tomorrow."