Hall of Fame catcher Gary Carter said doctors discovered four "very small" brain tumors after he had an MRI on Friday in Florida.
The 57-year-old Carter said he expects to learn more about his diagnosis when he is examined again Thursday at Duke Medical Center.
"My wife, Sandy, and our children and family thank you for your thoughts and prayers," Carter said in a release issued Saturday by the New York Mets and baseball's Hall of Fame. "We ask that you please respect our privacy as we learn more about my medical condition."
The statement did not say whether the tumors are malignant or benign.
Friends and former teammates were stunned when they heard the news.
"The last time I saw him a couple months ago, he looked well. He was Gary," said Mets first base coach Mookie Wilson, a teammate of Carter's in the 1980s. "He seemed himself. No indication anything was wrong.
"Whatever the situation may be, of course I wish him well. And I hope things turn out for the best."
Lee Mazzilli, another teammate on the 1986 Mets, was on his way to Yankee Stadium when his wife called with the news about Carter. He said he plans to reach out to Carter and his family when the time is right.
"You hope that it's not what you think it is," said Mazzilli, who works in corporate sales and sponsorships for the Yankees. "It breaks your heart. But I think if anyone has an optimistic outlook, he has one."
Fox broadcaster Tim McCarver, a Mets announcer in the 1980s, said he immediately thought of Carter's bright smile.
"The ear-to-ear smile that only Gary has. And obviously a smile is very important at a time like this," McCarver said. "I think everybody is kind of in that wait-and-see mode. ... But certainly he has occupied our thoughts since we heard the news."
McCarver was at Yankee Stadium preparing to broadcast the Subway Series game between the Mets and Yankees.
"It's tough to go on the air without thinking about him and being reminded once again how great the game is that it takes you away from the travails of life," he said.
Carter hit .262 with 324 homers and 1,225 RBIs in 19 seasons in the majors. The 11-time All-Star played his last game with the Montreal Expos in 1992 and was inducted into the Hall of Fame in 2003.
"I'm shocked at the news. It gives us all a sense of our mortality. Gary was a great teammate and naturally my prayers are with him and his family," Keith Hernandez said in a statement released by the Mets.
The effervescent Carter, nicknamed "Kid," is perhaps best known for helping the Mets win the 1986 World Series. He had 24 homers and 105 RBIs that year, then drove in 11 runs in the postseason.
"On behalf of the Mets organization, our thoughts and prayers are with Gary, Sandy and the entire Carter family," said Jeff Wilpon, the club's chief operating officer.