France defender Eric Abidal had been enjoying his best season ever with Spanish champions Barcelona, but is now preparing to go into hospital for surgery to remove a liver tumour.
The club said the operation would begin at 1500GMT on Thursday.
That was a day earlier than originally planned, for organisational reasons and not due to any deterioration in his condition, the club said.
Barcelona called for "the utmost respect for his right to privacy".
Sports daily Mundo Deportivo, quoting sources at the club, said Barca hope the tumour has been detected early enough.
Quoting doctors specialising in liver ailments, the paper said the organ is capable of recovering on its own and that if the tumour is benign, or non-cancerous, "Abidal could even return to training before the end of the season" in May.
Spanish media said the tumour was detected during a routine check, and the sports daily Marca said the player felt unwell on Tuesday morning.
The 31-year-old defender, capped 52 times for his country, has come a long way this season since the fiasco of France's World Cup campaign in South Africa.
Supported by coach Pep Guardiola, he has re-emerged strongly with Barca, with performances that convinced France coach Laurent Blanc to recall him to the squad, on the left side of the defence.
Under contract with Barcelona until 2012, Abidal has also adopted a place in central defence, despite having previously failed to convince in the role for France.
Recalled into the first team due to the long absence of Carles Puyol, the number 22 has drawn high praise from the fans at Camp Nou.
The news of the tumour is a blow for league leaders Barcelona, who are in the final straight of another magnificent season.
The club top the league standings, five points ahead of arch-rivals Real Madrid, have qualified for the final of the Spanish Cup and are still in the race for a fourth Champions League triumph.
Abidal, signed from Lyon in 2007, played in both legs of Barcelona's Champions League last 16 defeat of Arsenal.
Since news of his tumour emerged late on Tuesday, he has been showered with praise and words of support from his colleagues, as well as from his opponents.
"This is the most difficult moment that we have ever experienced in the changing room," said Barcelona midfielder Xavi of the squad's reaction to the news.
"Abidal will beat this because he a strong person, mentally one of the strongest that I know. These are difficult moments for him and his family.
"We encouraged him but it was he who gave us encouragement. He is an example for everyone," Xavi added.
"I want to show all my affection and support for Eric Abidal. I'm sure everything will be okay," Abidal's Barcelona team-mate, World Cup winner Andres Iniesta, said on his Facebook page.
"I hope that everything will be fine for Abidal and that he will return soon," said Real star Cristiano Ronaldo.