Carter, 57, released a statement through the Mets on Saturday explaining the situation as it currently stands.
"Earlier today, I learned that four very small tumors have been found on my brain, following an MRI on Friday at a West Palm Beach facility," Carter said. "I am scheduled to be examined further Thursday at Duke Medical Center, and we will learn more at that time about my diagnosis.
"My wife, Sandy, and our children and family thank you for your thoughts and prayers. We ask that you please respect our privacy as we learn more about my medical condition."
Mets chief operating officer Jeff Wilpon also released a statement about Carter's situation.
"On behalf of the Mets organization, our thoughts and prayers are with Gary, Sandy and the entire Carter family," Wilpon said.
According to the New York Daily News, which first reported Carter's condition, the former catcher had been experiencing headaches and forgetfulness recently before going for the MRI, which revealed the tumors. Carter has been coaching the baseball team at Palm Beach Atlantic University in Palm Beach, Fla.
Carter, inducted into the National Baseball Hall of Fame in 2003, played 19 years in the Majors with the Expos, Mets, Giants and Dodgers, batting .262 with 324 home runs and 1,225 RBIs.
An 11-time All-Star and three-time Gold Glove winner, Carter won a World Series ring with the Mets in 1986, when they beat the Red Sox in seven games. He delivered a two-out RBI single in the 10th inning of Game 6 to send the Mets to their improbable comeback victory, punctuated by Mookie Wilson's grounder going through Bill Buckner's legs.
Wilson is currently the Mets' first-base coach and was among many in the clubhouse concerned about Carter.
"I just found out about this today," Wilson said. "[My wife] Rosa and myself send out our thoughts and prayers to Gary, Sandy and the entire Carter family."
Former teammate Keith Hernandez also responded to the news.
"I'm shocked at the news," Hernandez said. "It gives us all a sense of our mortality. Gary was a great teammate and naturally my prayers are with him and his family."
"That's really sad," Twins manager Ron Gardenhire said prior to Saturday night's game against the D-backs at Chase Field. "Gary is a good friend and a former teammate of mine. I was honestly in shock when I saw it scroll across [the TV] today. Really sad."Gardenhire, a .232 lifetime hitting infielder, said he was a "futility player" with the Mets for two full seasons in the 1980s and parts of two others. Carter was traded by the Expos to the Mets for the start of the 1985 season and played five years in New York. Gardenhire played only 26 games for the Mets in '85, but Carter still made an impression. "He was really, really a nice young man," Gardenhire said. "When he came from Montreal to us, it was pretty special to have him around the clubhouse. He had a smile on his face most of the time. He was a pretty good guy."
Known as "The Kid" after he debuted at the age of 20, Carter was a two-time All-Star Game Most Valuable Player and was voted into the Hall of Fame on his sixth appearance on the ballot, gaining 78 percent of the vote.
John Schlegel is a national reporter for MLB.com. Barry M. Bloom contributed to this report. This story was not subject to the approval of Major League Baseball or its clubs.