The 27-year-old, who signed for Celtic in a club record pounds 4.5m deal in July 1996, was informed during the close season that urine and blood samples were "irregular", leading to fears that he could be suffering from a cancer- related illness.
In a statement released by Celtic yesterday, Stubbs admitted concerns had been raised but refused to confirm their precise nature. He has also appealed for privacy.
"Following standard tests after the Cup final, results have raised concern about a possible illness," said the former Bolton Wanderers player, who had been a target for Aston Villa towards the end of last season. "I will be having an exploratory operation [today]. I would appreciate if all contact regarding this matter is referred to the club and that my family's privacy is respected."
Celtic's chief executive, Allan MacDonald, said: "As soon as the matter was brought to the club's attention we have sought to give Alan all the support we can and ensure swift and professional medical advice."
Stubbs' agent, Paul Stretford, joined the chorus of voices wishing the player a speedy recovery. "I have spoken with Alan and he is very upbeat about the situation and will obviously follow the advice of the doctors and specialists who will be taking care of him," he said.
The discovery raises parallels with the British discus thrower Peter Gordon, who was banned for four years after producing only a third of the urine sample required after a major meeting in 1993. He was later reinstated after it was revealed he was suffering from testicular cancer.Reuse content