The Mets selected Muckenhirn's contract from Triple-A Syracuse prior to the second game of Monday's doubleheader with Atlanta.
The Mets will have Muckenhirn and Jose Butto -- who was designated as the 27th man for the twin bill -- available to provide length behind starter Tylor Megill for the second game after New York treated the opening game as a bullpen day, with five relievers covering nine innings in the 9-8 loss. Muckenhirn is in the majors for the first time in his career after he churned out a 0.77 ERA and 1.25 WHIP over 11.2 innings at Syracuse, though those ratios were supported by a poor 4:4 K:BB.