Welbeck suffered the injury after going up for a header half an hour into the contest with Arsenal’s medical staff immediately called onto the pitch by nearby players.
Oxygen was administered before the forward was carried off wearing a protective boot on his right leg.
It is another harsh blow for Welbeck who has battled through major injuries to both his knees during his time at Arsenal.
Welbeck scored the winning goal in the reverse fixture in Lisbon three weeks ago and was today named in Gareth Southgate’s England squad for games against United States and Croatia.
An official update from the club is expected later this evening.