Gary Cahill was treated this week for gastroenteritis, Chelsea head coach Antonio Conte said on Friday.
As Telegraph Sport revealed yesterday, 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 on Friday.
"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 this season.
"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.