Welbeck landed awkwardly after leaping in the air to meet a cross just after the half hour mark against Sporting, and needed oxygen before being stretchered off the pitch.
The 27-year-old has been troubled with knee problems in the past but has managed five goals in 16 appearances for the Gunners this term. "He's in hospital now and we'll see what happens", defender Rob Holding said.
He said: "This is the worst news tonight".
"His injury was hard for the mentality of the players because they looked at the injury".
"Danny is a tough lad, so if he's on the floor, I know it's something serious".
"He's been so unlucky with injuries over the past few years, but he's a good character and got a good set of lads around him so we'll lift him up and get him back as soon as we can".
"This injury is very big for him, for us and for everyone".
Given that Arsenal's goalless draw with Sporting Clube de Portugal was otherwise fairly routine, it was little wonder that Unai Emery's post-match press conference was dominated by questions about Danny Welbeck.
"We are going to wait for news on this injury, but we think it is serious".
The game was then stopped for several minutes after Welbeck contested a header with Gaspar before appearing to suffer an ankle injury on landing. "I think the players were concentrated for the match but when we finished the first thing was to ask about him".