Leeds winger Jack Clarke is "responsive" in hospital after being taken ill during his side's 1-1 Sky Bet Championship draw at Middlesbrough.
The 18-year-old, who had been substituted at half-time, complained of feeling unwell in the dugout and was then carried away on a stretcher while being given oxygen.
Leeds said in a statement: "Jack Clarke began to feel unwell during the second half of our Sky Bet Championship fixture with Middlesbrough and received medical attention.
"The player is responsive and attending hospital with the club's head of medicine and performance.
"On behalf of everyone at Leeds United, we would like to thank the medical staff from both clubs for their response to the incident."
After the lengthy delay, Leeds went on to score a last-gasp equaliser in the 12 minutes of stoppage time, and manager Marcelo Bielsa revealed after the game that the player had shown signs of improvement.
Bielsa 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 feeling better.
"He has been taken to hospital to see if he's alright."
Middlesbrough boss Tony Pulis said the disappointment of conceding a late equaliser paled into insignificance when compared to his concerns for Clarke.
Pulis said: "The more important thing is making sure the boy, fingers crossed, is OK and recovers because he is a very, very talented young player.
"All of our thoughts from Middlesbrough Football Club go to the lad Clarke."