Muno, 26, was hitless in nine at-bats during his second career big league stint, which lasted a little more than a week. Overall this season, he is 1-for-15, with the lone hit coming in his first career plate appearance.
The Mets could have spared Muno's roster spot and instead optioned Campbell, whose 0-for-23 stretch at the plate has dragged his average down to .179. Each were candidates at one point to replace injured third baseman David Wright, whose job has since gone to Ruben Tejada.
The Mets utilized a short bench frequently in April without many regrets, preferring the flexibility of an eight-man bullpen. This time, they are using it to extend their rotation from five to six.
Anthony DiComo is a reporter for MLB.com. Follow him on Twitter @AnthonyDiComo and Facebook, and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.