Stoke City's German defender Robert Huth is undergoing tests for suspected meningitis after falling ill on the club's US pre-season tour, the English Premiership side said on Wednesday.
The Potters said on their website stokecityfc.com that Huth was taken to hospital on Monday after his symptoms got worse over the weekend.
"It's a massive, massive worry for us but the most important thing right now is Robert's health and we are keeping our fingers crossed that he'll be okay," manager Tony Pulis said.
"We don't know the full extent of it at the moment but he'll be undergoing more tests, so hopefully we'll know more about the situation with him on Wednesday."
Pulis said that the 27-year-old's health was of prime concern, despite his condition raising doubts about whether he will be available for the new season that starts in 10 days' time.
The Britannia Stadium side are also waiting to hear about the recovery from injuries of a number of players, including record signing Peter Crouch, as well as midfielders Matthew Etherington and Marc Wilson.
Meanwhile, Stoke have finally been given clearance to complete their capture of United States international Geoff Cameron.
Cameron has been granted a work permit after Pulis and chief executive Tony Scholes successfully argued Cameron's case at an appeals hearing at the Etihad Stadium in Manchester.
Houston Dynamos utility player Cameron is currently a first choice for the US national side.
Scholes said: "Geoff has also been in the MLS All Stars XI three times in the past five seasons, voted by his peers as one of the best.
"In those other two seasons when he wasn't selected, one was his rookie year and the other he was injured."