N'Golo Kante did not feature in Chelsea's match against Premier League leaders Manchester City at the weekend after fainting at the club's training ground, reports said on Tuesday.
The Daily Telegraph newspaper said shocked team-mates feared the France international had suffered a heart problem when he briefly passed out after training on Friday.
It said a Chelsea doctor was on the scene within seconds of Kante slumping to one side in the dressing room at the club's Cobham training ground southwest of London.
The 26-year-old was given the all-clear after cardiology tests, the report said, and Kante travelled with the squad to Manchester for Sunday's match. He was cleared to play but felt unwell on Sunday morning.
The midfielder was back in training on Monday and is expected to be available for Saturday's Premier League home game against Crystal Palace.
Kante's health scare came two days before Fiorentina captain and Italy international Davide Astori, 31, was found dead in his hotel room in Udine.
Chelsea boss Antonio Conte, who coached Astori when he was in charge of the Italian national team, said the player's death was a "tragedy".