Sergio Aguero was taken to hospital after complaining of dizziness during the first half of Barcelona's 1-1 draw with Alaves on Saturday, interim head coach Sergi Barjuan confirmed.
The Argentine was absent for the first two months of the season as he recovered from a nagging calf injury, finally making his club debut in mid-October against Valencia.
But he was in clear discomfort on the pitch before coming off late in the first half, later leaving the stadium as a precautionary measure.
What happened?
Almost 40 minutes were on the clock at Camp Nou when Aguero pulled up in discomfort.
The forward was left hurt by a clash in the area and appeared to have trouble breathing, pointing to his chest as he doubled over.
After an assessment from the Barca medical team, he left the game almost in tears, with Philippe Coutinho chosen to replace him in the Catalans' line-up.
The Argentine left Camp Nou in an ambulance, following medical protocol in such cases.
In the second half he was joined on the sidelines by Gerard Pique, who limped off to be replaced by Clement Lenglet with just over 20 minutes still to play.
The club later confirmed that he had undergone a cardiac exam after complaining of discomfort in his chest.
"Aguero was taken away," Barjuan explained to Movistar following the game.
"I have asked him. He told me he was a little dizzy. I have found out now that they have taken him to the hospital to see what he really has. I do not know any more."
The club later confirmed that Pique was removed from the game with a calf strain.
Barca were taking on Alaves in the club's first game since Koeman's sacking on Wednesday.
The bigger picture
Prior to Saturday's scare, injuries had proven a constant concern for Aguero over the past few years.
In 2020-21 the then-Manchester City star was restricted to just 12 Premier League appearances as knee and hamstring problems, as well as a long battle with Covid-19, cast a shadow over what proved to be his last season at the Etihad Stadium.
Fitness issues have also hindered his start at Camp Nou, although he was able to mark his first Clasico with a goal as he netted a late consolation for Barcelona off the bench against Real Madrid last weekend.