Leeds winger Jack Clarke was taken to hospital after complaining of illness in the second half of the Sky Bet Championship 1-1 draw with Middlesbrough.
There was a lengthy delay after the 18-year-old, who was substituted at half-time, was taken ill in the dugout and then stretchered off while given oxygen.
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 promising teenager had shown signs of improvement before being 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 feeling better. He has been taken to hospital to see if he's alright."
Leeds later added in an official 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."