On Saturday, Darling, a longtime SNY analyst, said he had a "large mass in my chest" and that he hoped to return to the broadcast booth in May.
The 58-year-old Darling said a series of tests revealed the mass. He said doctors have told him that "if there are no complications I should be back on air talking baseball sometime next month."
The Mets said doctors had been monitoring Darling over the past few months.
SNY studio analyst and former Mets player Todd Zeile is filling in for Darling during his medical leave, according to the Mets.
Darling was sued this month by former Mets teammate Lenny Dykstra over a passage in Darling's new book that accused the outfielder of directing racist comments toward Boston Red Sox starter Oil Can Boyd during the 1986 World Series.
The Associated Press contributed to this report.