Nacional defender Juan Izquierdo collapsed on the pitch and was taken to hospital after suffering an irregular heartbeat during the second leg of a Copa Libertadores round of 16 tie against São Paulo at Brazil's Morumbi stadium on Thursday.
Izquierdo had replaced Sebastián Coates for the Uruguayan club at halftime but fell to the ground unconscious in the 84th minute without any contact from another player.
Players from both sides immediately called for medical assistance. Still unconscious, the 27-year-old was taken off the pitch in an ambulance to the applause of the fans.
"Juan Izquierdo suffered an irregular heartbeat during the match against Sao Paulo," Nacional posted on social media platform X.
"He is being treated at the Intensive Care Unit of the Albert Einstein Hospital. He is currently in stable condition and remains under observation."
Sao Paulo wished Izquierdo a speedy recovery in a post on X.
The match continued with goals from Damián Bobadilla and Jonathan Calleri giving Sao Paulo a 2-0 win which, after a scoreless draw in the first leg, sent them through to an all-Brazilian quarter-final clash against Botafogo.