Leeds United youngster Jack Clarke was treated by paramedics on the bench during their Championship game at Middlesbrough.
The game was paused while medics provided emergency assistance, resulting in 12 minutes of added time being played after the lengthy delay.
Clarke, 18, was taken to hospital after the worrying scenes at the Riverside Stadium on Saturday lunchtime.
He had started the game, which kicked off at 1pm, but was substituted at half time and was sat on the bench with his team mates when the incident occurred.
Leeds manager Marcelo Bielsa confirmed after the match that it was the teenager who 'didn't feel well', and has been taken to hospital.
He said: 'What we know is that he didn't feel well. We don't know the reasons yet, but when he was taken to the dressing room he was better. He was taken to hospital to see if he is alright.'
Bielsa also went on to confirm that Clarke had been taken off for tactical reasons, rather than because of any illness or injury.
Clarke was heavily linked with a move away from Leeds in January, with Manchester City, Arsenal and Tottenham all among clubs keeping tabs on the teenager, valued at around £12million.
Leeds rescued a 1-1 draw away to their promotion rivals after Kalvin Phillips scored a 101st-minute equaliser.
More to follow.