Gary Cahill was treated this week for gastroenteritis, Chelsea head coach Antonio Conte said today.
The 31-year-old was admitted to hospital on Tuesday after training and released on Thursday following treatment and tests.
"He arrived at the training ground with a bit of fever," Conte said.
"Our doctor preferred to bring him to the hospital and to check his condition.
"Not a serious problem, but now Gary is getting better and is improving a lot."
Chelsea play Tottenham at Wembley in Saturday's FA Cup semi-final.
Conte appears resigned to being without his on-field captain.
"If you ask me for tomorrow, my answer is it is very difficult," Conte added. "This period is not a lucky period for us."
Spurs are four points behind the Blues at the Premier League summit with six games remaining after Chelsea's loss at Manchester United last Sunday.
Tottenham boss Mauricio Pochettino said his side were underdogs for the Wembley clash.
But Conte reckons it is time for Spurs to shed their self-titled underdog status.
"Tottenham now is a really great power in English football," Conte said.
"There is a moment you have to finish considering yourself an underdog. I think this is the right moment.
"This is the third year for them (under Pochettino).
"Tottenham is a great team. They're showing for the second consecutive year to fight for the title."