Barcelona striker Sergio Aguero was taken to hospital after suffering "chest discomfort" and complaining about feeling dizzy during his side's 1-1 La Liga draw with Alaves on Saturday.
Aguero, 33, appeared to signal to the bench before going down clutching his chest just before half time. He received treatment for several minutes before being helped off the pitch.
"Sergio Aguero was struggling with chest pain for much of the first half and was replaced by Philippe Coutinho at half time and then taken to hospital for cardiac analysis," Barcelona said.
The Argentine was making his first start for the club since his move in the summer.
Aguero joined Barca on a two-year deal after 10 years at Manchester City, a spell which saw him become their all-time leading goalscorer with 260 goals and saw them crowned Premier League champions five times.
"I was told he was feeling a bit dizzy. I have just learnt that he was taken to hospital and I can't say much more until I know," Barcelona's caretaker coach Sergi Barjuan said.
With Barcelona great Xavi expected to become permanent successor to the sacked Ronald Koeman, B team coach Barjuan took control of Saturday evening’s game.
Real Madrid reclaimed top spot – eight points ahead of ninth-placed Barca – following a 2-1 triumph at 10-man Elche.
Vinicius Junior claimed both Real goals, either side of Raul Guti being dismissed for a second bookable offence, before Pere Milla claimed the hosts’ late consolation.
Sevilla are level on points with Carlo Ancelotti’s men – and Real Sociedad – after strikes from Diego Carlos and Lucas Ocampos secured a 2-0 win over Osasuna, while Valencia defeated rivals Villarreal by the same scoreline thanks to Hugo Guillamon’s first-half finish and a Carlos Soler penalty.