Unai Emery admitted the injury suffered by Danny Welbeck during Arsenal's goalless draw with Sporting Lisbon had an adverse effect on his players.
The Emirates Stadium stalemate was enough to seal progression into the latter stages of the Europa League - as they now target top spot in Group E.
A forgettable game was overshadowed by a first-half injury to England forward Welbeck, who was taken to hospital after being carried off on a stretcher.
Many of Welbeck's team-mates appeared distressed as he received lengthy treatment and needed an oxygen mask after what appeared to be an awkward landing as he challenged in the air for the ball.
Emery, whose side extended their unbeaten run to 15 games by shutting out Sporting, conceded the situation weighed heavy on his players.
"It's the worst news tonight, this injury. It's clear that we are with him in these minutes," he said.
"He was working well and he was having a good match. He had some chances too. His injury was with the mentality of the players. They saw it was a big injury.
"When you are playing football, when you are playing in competitive matches, it can happen. You don't want it to happen but this is our profession.
"Now he is at the hospital. The news is that we are going to wait - but we think it's a serious injury."
Welbeck has endured a number of serious injuries during his career and it remains to be seen how long his latest setback will leave him on the sidelines.
The 27-year-old is out of contract at the end of the season and Emery would not be drawn on whether Welbeck's future is at Arsenal.
"He's a very good person," he said.
"His commitment with us is very big. He's helped us with his behaviour, with his quality and his capacity.
"It's a very big injury for him, for us, for all people who like and love him as a person."
Sporting captain Nani, a former team-mate of Welbeck at Manchester United, was one of the first one the scene when it was clear he was in trouble.
After the game, the 31-year-old offered his support to Welbeck: "It was a very sad moment," he said.
"I feel very sorry for him, he is a great person. A good lad and a good friend. When that kind of moment happens in the game you feel very sad.
"I don't know what to say, I just want him to recover well and I hope it is nothing serious but you can see something happened there so I wish him all the best and a very fast recovery."