Stiliyan Petrov has been diagnosed with an acute form of blood cancer - leukemia.
"Stiliyan is a favorite of many, so he will receive from the club all the love and support hoping for the best result. At the moment, we ask to give the footballer privacy - we will announce all the information as soon as we receive it," the statement said.
Petrov moved to England in 2006 when he was acquired for 7 million euros from Glasgow Celtic. In 219 matches, he managed to score 13 goals.
"We pray to God that he recovers," wrote Aston Villa striker Darren Bent. "Very sad news about the team captain. Our thoughts are with him and his family."
Celtic coach Neil Lennon added: "He will overcome this obstacle like all others that have arisen in life. Stiliyan's heart is like that of a lion."