Arsenal fear forward Danny Welbeck has suffered a "very big injury" after he was carried off on a stretcher during his side's Europa League game against Sporting Lisbon on Thursday.
It ended goalless at Emirates Stadium as the Gunners secured a place in the last 32, but the match was overshadowed by the 27-year-old England international's ankle injury.
"He is at the hospital. The news is that we are going to wait, but we think it's a serious injury," said Arsenal boss Unai Emery.
"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."
Welbeck was hurt in the 25th minute when he jumped for a header and landed badly on his right 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.
Arsenal midfielder Henrikh Mkhitaryan with team-mate Welbeck
Some of Welbeck's team-mates were also visibly affected by the incident.
"They saw it was a big injury," added Emery.
"When you are playing football, when you are playing in competitive matches, it can happen. You don't want it to happen, but this is our profession."
Welbeck has scored five goals this season, played for England against Spain in September and had been named in Gareth Southgate's latest squad for the friendly with the United States and the Nations League match against Croatia.
Former Arsenal defender Martin Keown, working as a pundit for BT Sport, said: "Danny has had two major injuries to his knee and he tried to plant his foot and got it stuck behind him.
"He is a great player and good for that group as he is 27, one of the older players. He will be a big loss for them. It looks like he will be out for a while."
Arsenal midfielder Mesut Ozil, who did not feature in the match, tweeted his support for Welbeck
Sporting ended the game with 10 men when ex-Barcelona defender Jeremy Mathieu was shown a straight red card after fouling Pierre-Emerick Aubameyang when he was clean through on goal - although the hosts could not find a winner.
The result, along with a 1-0 win for Azerbaijani side Qarabag over Vorskla Poltava of Ukraine in the other Group E match, was enough to take the Gunners through to the knockout phase. They join Chelsea in the last 32, who had secured their place in the next stage with a 1-0 away win at BATE Borisov.
Arsenal are fifth in the Premier League, unbeaten in 15 games and have 10 points from their opening four Europa League games.
Emery's side are three points clear of Sporting Lisbon in second and will win the group with victory at Vorskla Poltava on Thursday, 29 November.
After winning their opening three Europa League games, Emery was able to make nine changes from the side that drew 1-1 with Liverpool on Saturday. Only Rob Holding and Henrikh Mkhitaryan kept their places, with Mesut Ozil, Alexandre Lacazette, Granit Xhaka and Hector Bellerin left out completely.
Top scorer Aubameyang started on the bench but came on as a first-half substitute for Welbeck.
Aubameyang hit the bar early in the second half and later saw his goal-bound shot blocked by former Liverpool defender Sebastian Coates.
Mathieu was dismissed in the 87th minute for fouling the Gabon striker, who fired the resulting free-kick over the bar.
There was another injury problem for the Gunners in the second half as right-back Stephan Lichtsteiner pulled up with what appeared to be a hamstring problem on a mixed day for the club.
Henrikh Mkhitaryan had one shot on target, an 88% pass accuracy and made 75 touches
Arsenal boss Emery said: "Every match we want the best performance from every player. Some players need to take responsibility and confidence, and there were a lot of positives tonight.
"The point isn't enough for us, but at the moment we are on target to be first in the group.
"Sporting are a very big, competitive team and organised. We needed to be calm.
"It was difficult to create big chances but it wasn't enough to win."
Arsenal return to Premier League action on Sunday when they entertain Wolves (16:30 GMT kick-off).
The Gunners are next in Europa League action on 29 November, away at Vorskla Poltava (17:55), with Sporting travelling to play Qarabag at the same time.