Danny Welbeck's latest knee injury has left the forward "devastated" and Arsene Wenger baffled, the Arsenal manager has said.
Welbeck, 25, was ruled out for nine months following surgery on Wednesday to repair cartilage damage in his right knee after suffering the injury against Manchester City on Sunday.
Welbeck only returned in February from a similar layoff after surgery on his other knee, but had scored five goals and looked set for a place in Roy Hodgson's England squad for Euro 2016.
"We're all devastated, and Danny even more so," Wenger told Arsenal's official website. "We can only feel sad and support Danny now in order to get him back."
The latest injury is completely unrelated to Welbeck's previous knee problem and was sustained as he went in for a tackle near the sideline 20 minutes into the 2-2 draw at the Etihad Stadium.
"It was basically from an anonymous tackle. The difference between the tackle and the severity of the injury is baffling," Wenger said. "We don't understand that but we have to accept the verdict and we have to live with it.
"Danny has to deal with it unfortunately. We just have to give him the maximum support we can and show him that we trust he can come back."
Welbeck had given the Arsenal attack a much-needed spark since his return, which included the last-minute winner against Leicester in his first appearance of the season in February.
But Wenger had used him sparingly of late, in part to avoid pushing his body too hard after his previous 10-month spell out on the sidelines.
"I pushed him in and out of the team because I knew he'd been out for a year. Sometimes for the big games I kept him out [before] to recover from the efforts he has made," Wenger said.
"Sometimes people accused me of being too cautious but I knew that he did not play for a year and the intensity of the Premier League is so exceptional that you have to be cautious. The injury was accidental. I don't think there's anyone to blame, not our opponents or medical staff. It was completely an accident."