Hartlieb was recalled from Triple-A Syracuse on Friday.
Hartlieb was sent down Monday, but he'll provide an additional arm out of the Mets' bullpen after right-hander Jake Reed (forearm) was placed on the 10-day injured list Friday. The 27-year-old has made three relief appearances with the Mets this year, and he's allowed seven runs on seven hits and six walks while striking out five in 4.1 innings.