The Arsenal striker landed awkwardly after going for a challenge in the 25th minute and required immediate attention.
The 27-year-old has been riddled with fitness problems since joining the Gunners from Manchester United for a fee of £16million four years ago.
Unai Emery’s men earned a point in a drab goalless draw against Sporting.
Aside from Welbeck, Stephan Lichtsteiner came off with a hamstring injury to cast a further shadow on proceedings.
However, the Gunners have secured their progress to the knockout stages of the competition as Emery seeks to win a major piece of silverware in his debut campaign in charge of the north London outfit.
Has Danny Welbeck played his last game for Arsenal?
Welbeck was rushed to hospital after the match and Emery reckons he has broken his ankle.
The former England striker required was given oxygen on the pitch to aid his breathing.
As of yet, there has been no further update from the club.
It is not yet clear how long Welbeck will be sidelined for.
When asked for his thoughts on the injury, Emery said: “It’s his ankle. He’s broken something in his ankle.”
When discussing the players’ reaction to the injury, he said: “When a player has an injury like that the players look and in your mind you feel for the player.
“Then the players concentrated on the match and we forgot.
“When we finished the match the first thing was to ask about him.”
However, with Welbeck out of contract at the end of the season, it could be the last time he plays for the club.