Milledge, 21, was batting .239 with four home runs and 18 RBI in 40 games with New York.
He is expected to be recalled when rosters expand to 40 players on Sept. 1, but the Mets needed to clear a roster spot for pitcher Brian Bannister, who is slated to pitch Friday against the Philadelphia Phillies.
Considered to be the Mets' top prospect, Milledge appeared to take the news hard. He was in a somber mood following New York's 6-2 victory over the St. Louis Cardinals.
"You don't know if it's just going to be a week or so," Milledge said. "They haven't told me anything. Nothing is guaranteed."
Mets manager Willie Randolph confirmed that Milledge will return when rosters expand. In the meantime, Milledge - drafted as a centerfielder in 2003 - will be asked to play mostly left field at Triple-A.
"I'll play left," Milledge said. "It's cool."
Recalled to the major leagues on Aug. 1 for the second time this year, Milledge was batting .250 with one home run and six RBI in 16 August games.
With Cliff Floyd on the disabled list, Milledge and Endy Chavez had been filling in at both corner outfield positions, but the team's acquisition of Shawn Green made Milledge extraneous on the 25-man roster.
Green made his Mets debut Thursday at Shea Stadium, going 1-for-3 with a walk and one RBI.