France international Abidal, 31, will go in for surgery on Friday and Kaka was one of many stars to pay tribute to a rival.
"What sad news about Eric Abidal, tumour in the liver!" Kaka, a committed Christian, said on Twitter. "Let's pray for him. God bless!!"
Real team-mate Cristiano Ronaldo also commented on the news, tweeting: "I hope it all goes well with Abidal, and that he gets back soon."
Abidal to have surgery on liver tumour
Arsenal captain Cesc Fabregas, whose team went out to Barcelona in the last 16 of the Champions League and has been linked with a return to his hometown club, said: "Much encouragement and a lot of strength to Eric Abidal. Bad news for football."
Barca announced the news on Tuesday night.
Abidal joined Barca from Lyon in 2007 and has made over 140 appearances for the club in all competitions.
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