DeJean, 33, who has also pitched for the Colorado Rockies, Milwaukee Brewers and St. Louis Cardinals in his Major League career, was 0-5 with a 6.13 ERA in 37 games with Baltimore this season. He will join the Mets on Tuesday.
"I'm excited to come to New York and be in the middle of a pennant race," DeJean said in a statement. "I played for [Mets bench coach] Don Baylor in Colorado and know a lot of guys in the clubhouse. I can't wait to get there."
Though DeJean pitched effectively last season for Milwaukee, recording a team-high 18 saves in 58 games as the Brewers' closer before he was dealt to the Cardinals on August 22, his first season in the American League was nothing short of a disaster.
Duquette analyzed DeJean's struggles – opposing hitters were batting .308 off of the righthander – by pointing out that DeJean's command on his fastball has been lacking.
The Mets are hoping that Peterson can help corral DeJean's heater while complementing the rest of his repertoire. Duquette said that DeJean has a solid slider and also throws a good forkball and change-up.
"He just gives us another option," Mets manager Art Howe said. "I think working with Rick might be able to iron some things out."
"He's a project worth taking a risk on," Duquette added.
Garcia, 28, had become a superfluous part with the Mets in the weeks following the club's acquisition of outfielder Richard Hidalgo from the Houston Astros. In 62 games with New York this season, Garcia was hitting .234 (45-for-192) with seven homers and 22 RBI.