Arsenal believe Danny Welbeck suffered what may be a “very big injury” in their home draw against Sporting Lisbon in the Europa League.
The 27-year-old forward was carried off on a stretcher in the first half after jumping for a header and landing badly on his ankle.
He received oxygen and had his leg in a protective brace as he was taken off the pitch, with up to 10 medical staff around him as he received treatment.
Gunners boss Unai Emery said: "He is at the hospital. The news is that we are going to wait, but we think it's a serious injury.
"Every injury is different. This could be a break of something in the ankle.
"It's clear that we're with him in these minutes. He's a very good person and his commitment with us is very big.
"It's a very big injury for him, for us, for all people who like and love him as a person."
The draw saw Arsenal qualify through to the last-32 stage of the tournament.