N'Golo Kante passed out in front of his team-mates after a training session, just two days before Chelsea were due to face Manchester City.
The France midfielder was cleared to play in Sunday's 1-0 defeat at City, but felt ill and he was left out of the game as a precaution.
Chelsea staff quickly attended to Kante on Friday at the training ground and then had a specialist conduct a series of cardiology tests, which have allayed fears of any heart problems.
Kante's collapse is thought to be a one-off and he is medically cleared to come back into the side against Crystal Palace on Saturday.
Danny Drinkwater and Cesc Fabregas played in Kante's absence at the Etihad Stadium as the defeat left Chelsea five points adrift of the top-four places in the quest to qualify for the Champions League.
Former Leicester midfielder Kante's collapse came just 48 hours before the death of Fiorentina captain Davide Astori.
Chelsea manager Antonio Conte spoke of his sadness at hearing the news about Astori, who he worked with at the Italian national team, while Marcos Alonso also spoke about losing a former team-mate after the City defeat.
“The truth is I still can’t believe what has happened," Alonso said. "But all I can do is give all my support to the family and to Fiorentina.
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“He was an outstanding guy, and a very healthy guy, so I still can’t believe that this has happened.
“It’s been a difficult day today, I am very sad for what has happened but when you are on the pitch you just have to try not to think about things away from the pitch."