Danny Welbeck was taken to hospital with Arsenal fearing a dismissal for serious injury.
The Arsenal striker was the victim of an x-ray after suffering a leg injury during the draw between the Europa League and Sporting Lisbon.
Welbeck headed for the lead with Bruno Gaspar in the first half and landed awkwardly before immediately reporting the treatment.
Medical personnel rushed to the field and administered gas and air to the attacker while being comforted by Henrikh Mkhitaryan, captain of Sporting and his former teammate. Manchester United, Nani.
Arsenal confirmed that Welbeck had suffered a serious ankle injury and went to the hospital for tests.
Unai boss Emery said, "It's his ankle, he broke something in his ankle.
"This is the worst news tonight, we think it's a serious injury, it's clear he was playing well and had opportunities.
"His injury also affected the mentality of the players as they could see that it was a serious injury.When we finished the match, the first thing to do was to ask about him.
"He was taken to the hospital, we will wait."
The setback could mean that Welbeck played his last game for Arsenal, with his contract in place at the end of the season.
If the injury is as serious as it is feared, he will face a race against time to return before the end of the season.
Welbeck was also linked to a January move with Crystal Palace, which would be very interested in the striker.
The striker has had a nightmare with injuries since his transfer from Manchester United to the United Arab Emirates.
He missed about eight months of action and Euro 2016 after suffering a knee injury at the end of the 2015-16 season.
But Welbeck has regained form in the last 12 months and has carved a place in the English team for the World Cup.
He had also been named in the Gareth Southgate team for the upcoming matches with Croatia and the United States earlier on Thursday.
Skipper Petr Cech said: "Danny [Welbeck]It looks like a serious injury that's not nice to see and our thoughts are going towards him. "
Alex Iwobi added, "It was a mental challenge, we will wish him the best and encourage him to come back."