Victor Lindelof went down under no contact and was quickly substituted in Manchester United’s Saturday night match in what appeared to be an unexplained medical issue.
With the score locked at 0-0 away to Norwich Ralf Rangnick was chasing a winner in his second Premier League match at Carrow Road.
But it was in defence he was forced into a change when United physios grew concerned for Swedish defender Lindelof.
Team-mates were quickly on the scene as the centre-back was seen on his haunches and pointing to his chest.
Midfielder Fred quickly signalled for a substitution to be made and he was replaced by Eric Bailly after being assessed, though appeared to have been keen to try and play through the problem that was bothering him.
But common sense prevailed when the call was taken out of his hands for the remainder of the game and he headed up the tunnel.
Cristiano Ronaldo then scored the winner with a penalty with 15 minutes remaining.
United captain Harry Maguire told Sky Sports: "He's in the changing room, he felt a bit of discomfort, I don't know too much detail.
"You're probably going to find out more in the next few days but he seems fine in there, he just felt a bit of discomfort and we all wish him well."
Keeper David De Gea added: "I hope he's completely fine.
"It was like breathing, difficulty breathing, he was feeling something strange so the game, the football doesn't matter, it's life, it's to be safe."
Finally manager Ralf Rangnick provided the most reassuring update as he said: "He had a collision, he can't even remember what it was but he was struggling to breathe.
"He felt pain in his chest but right now they've checked him, they did all the tests and everything seems to be OK."
