The 2005 first-round draft pick was optioned to Triple-A Norfolk on Wednesday and was replaced on the roster by left-handed reliever Royce Ring.
"I guess of the guys they are considering, I did the worst of all of them," said Pelfrey, who gave up four runs on seven hits in six innings Tuesday at Florida. "They told me I had to get more consistent with my secondary stuff and execute my pitches better."
Pelfrey was 2-1 with a 5.48 ERA in four starts.
The Mets had been going with a six-man rotation, but manager Willie Randolph said he'd go back to five after the Marlins series.
John Maine, who has thrown 17 consecutive scoreless innings over his last three games, will take over as the fifth starter.
REPLAY: The Mets continued their habit of setting an early tone by scoring four runs in the first inning Wednesday, and they held on for a 6-5 victory over the Marlins.
The Mets lead the major leagues with 98 first-inning runs.
Steve Trachsel started and gave up three runs in 5 2/3 innings. The bullpen showed its vulnerability without Duaner Sanchez by giving up two runs in the seventh, but Aaron Heilman and Billy Wagner worked the eighth and ninth, respectively, to close out the game.
RHP Mike Pelfrey was optioned to Triple-A Norfolk, where he will work on developing his breaking pitches and changeup. Pelfrey was 2-1 with a 5.48 ERA in four starts for the Mets. LHP Royce Ring was recalled to replace him. Pelfrey's demotion means RHP John Maine will assume the role as the fifth starter.
C Paul Lo Duca extended his hitting streak to 11 games with a first-inning single Wednesday night at Florida.
LHP Oliver Perez, obtained along with RHP Roberto Hernandez for RF Xavier Nady from Pittsburgh on Monday, threw a bullpen session for pitching coach Rick Peterson on Wednesday. "He has a live arm," Peterson said. "He has a lot of movement on his pitches."
OF Endy Chavez started over Lastings Milledge in right field Wednesday night.
RHP John Maine assumed the fifth starter role with the demotion of RHP Mike Pelfrey to Triple-A Norfolk. Maine will start Sunday against Philadelphia in New York.
The Mets won their 23rd one-run game of the season, most in the major leagues, by beating Florida 6-5 Wednesday night.
The Mets scored four runs in the first inning Wednesday at Florida, giving them 98 first-inning runs for the season, tops in the majors.