Leathersich, 25, underwent Tommy John surgery on July 30, so he is not due to return until next summer.
The Mets have a shortage of left-handed relievers, but chose anyway to remove Leathersich to clear a spot ahead of Friday's deadline to add prospects to the 40-man roster to shield them from the Rule 5 draft.
Leathersich was drafted in the fifth round in 2011 out of UMass-Lowell. He had a 2.31 ERA in 17 major league relief appearances.
Leathersich has an amazing rate of 15.2 strikeouts per nine innings during his minor league career, but also was prone to walk batters.