Danny Welbeck's season - and with it probably his Arsenal career - is all-but over, the Gunners fear.
Welbeck went up for a first-half header against Sporting Lisbon's Sebastian Coates and landed awkwardly on Thursday night.
The England international was carried off the pitch on a stretcher after requiring oxygen on the pitch.
But Mail Online says the damage done is significant and doctors are concerned Welbeck has suffered a break, dislocation and ligament damage.
If that proves to be correct, he won't play again this season.
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That would also likely mean the end of his Emirates career with the former Manchester United forward out of contract in the summer. There are no talks planned regarding a new deal.
An announcement is expected to confirm the news within three days.
Manager Unai Emery fears the worst and said after the game: "We think it's a serious injury.
"Every injury is different. He broke something in his ankle and it's a different injury than another.
"Now he's at the hospital and the news is at the moment we are going to wait, but we think this is a serious injury."
Defender Stephan Lichtsteiner is also facing four weeks out after sustaining a hamstring injury.
