Gary Cahill is a major doubt for Chelsea's FA Cup semi-final with Tottenham at Wembley on Saturday after being admitted to hospital this week, according to reports.
Several national newspapers have reported that Cahill was admitted to hospital on Tuesday after training and released on Thursday following treatment and tests on a 'mystery illness'.
Chelsea head coach Antonio Conte is scheduled to speak at a pre-match media conference at 1415 BST on Friday afternoon, where he will be questioned about Cahill's condition.
Cahill played in Chelsea's loss at Manchester United last Sunday, when the Premier League leaders' advantage over nearest rivals Spurs was cut to four points. There are six games remaining.
Goalkeeper Thibaut Courtois missed the United clash with an ankle injury and Marcos Alonso withdrew during the warm-up at Old Trafford due to a stomach bug, the Daily Telegraph said.
Victor Moses and Diego Costa played despite a similar bug, but Cahill's illness is unrelated, the Telegraph added.
Conte is unlikely to use the illness and injuries as explanations for the loss to United.
Chelsea are seeking a win over Tottenham which, as well as securing a place in the FA Cup final, would boost morale and confidence for the Premier League run-in
The Blues have been fortunate with injuries and illness this season, but the timing of the current situation is far from ideal.
Cahill has been Chelsea's on-field captain for much of the campaign, with John Terry out of favour.
The 31-year-old former Aston Villa and Bolton centre-back has 53 England caps and captained the Three Lions in March's friendly defeat to Germany.
Cahill has also been a key figure in Chelsea's defence this season, playing in a three-man defensive unit with Cesar Azpilicueta and David Luiz.
On Thursday, Cahill and Luiz were named in the Professional Footballers' Association team of the year on Thursday.
Terry, Kurt Zouma and Nathan Ake are in contention to step in at Wembley should Cahill be ruled out.
Terry and Chelsea announced this week that the 36-year-old defender will be leaving Stamford Bridge after 22 years at the end of this season.