Aston Villa captain Stiliyan Petrov has been diagnosed with acute leukaemia, the Premier League club announced on Friday.
Petrov's illness was discovered in medical tests carried out on the 32-year-old Bulgarian international after he came down with a fever following last Saturday's defeat at Arsenal, the club said.
"We expect to learn more about Stiliyan's situation in due course and we have moved quickly to support him and his family," the statement said.
"During this time we ask that Stiliyan's privacy is placed ahead of all inquiries and trust that we will share information as we receive it.
"Stiliyan is cherished by many and he will get from Villa every ounce of love and support that we have to help bring this to a positive conclusion."
Petrov joined Aston Villa in 2006 after a seven-year spell at Celtic.