Gee allowed a career-high-matching eight runs in 3 2/3 innings on Sunday against the Atlanta Braves in a spot start. That lifted his season ERA to 5.90.
"One of the reasons for making this decision today is that Dillon would not be available to us in the pen as of today, given that he pitched yesterday and we used a good part of our pen yesterday," Mets GM Sandy Alderson said. "We needed the additional arm immediately, and this is consistent with our desire to put the strongest possible 25-man roster together each night."
While Gee spent time on the disabled list with a mild groin strain last month, rookie Noah Syndergaard was promoted from Triple-A Las Vegas and took hold of the rotation spot.
The Mets initially contemplated a six-man rotation upon Gee's return, but quickly abandoned the concept. Gee did get a spot start on Sunday with the Mets looking to give their regular five starters extra rest, but flopped. He primarily would have been assigned to the bullpen.
Gee is earning $5.3 million this season and has one more year of arbitration eligibility before free agency. The DFA process gives the Mets a week to find a trade partner before having to put Gee on waivers for the purpose of sending him to Triple-A Las Vegas.
Reliever Akeel Morris was promoted from Class A St. Lucie to take a bullpen role, at least for the short term.
The Mets still plan to use a spot starter on occasion to give their five rotation members extra rest and to conserve their innings. Left-hander Steven Matz, whose 2.30 ERA ranks second in the Pacific Coast League, now becomes the primary candidate for those assignments.
"Right now we have no plans on sticking a sixth guy in there [for a] spot start," Mets manager Terry Collins said. "We're gonna stay with the five-man [rotation], and if we need to make an adjustment we'll do it."
Information from ESPNNewYork's Kieran Darcy was used in this report.