Martinez hit .194 with five RBIs in 18 games after being called up on May 26 to replace outfielder Ryan Church.
Martinez struggled early with the big club, going hitless in his first two starts and getting booed at Citi Field for not running out a dropped popup. Though he showed flashes of his promise during his three and a half weeks with the Mets, he never found a consistent rhythm at the plate.
"From a mechanical or a fundamental point of view, what they were able to do to Fernando here is to control his front foot," manager Jerry Manuel said. "He has to control that himself, so the pitcher doesn't keep him off-balance."
Manuel added that Martinez looked more upright in his stance than he did in years past and in Spring Training.
"When he came, I saw more of a slugger than a hitter," Manuel said. "That's tough for a young player."
The 20-year-old did not hit a home run with the Mets and went 4-for-26 in the team's last seven games. Church's return to health and the Mets' return to National League rules during Interleague Play mean there is no longer a spot in the lineup for Martinez.
Manuel was still impressed by his prospect.
"What I saw in Fernando was a young player that I really believe can handle the situation, and I really believe will be a part of us doing well or doing what we need to do at some point this season," he said.
Tim Britton is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.