"The only way you're going to find out and the only way they're going to get better is to be used," Collins said. "He's here to face some lefties. He's going to face lefties."
Leathersich, 24, appeared in two games during his first stint with the Mets, pitching one scoreless inning. A Minor League strikeout specialist, the left-hander punched out 20 batters over 11 1/3 innings at Triple-A Las Vegas.
Though the Mets also considered promoting right-handers Chasen Bradford or Zack Thornton to replace Carlyle, Leathersich -- the organization's No. 19 prospect, per MLB.com -- held an advantage due to his status on the 40-man roster.
"He's been pitching well," Collins said. "He's been throwing balls over the plate. His numbers say he strikes guys out, so you know what? You've got to trust him, and then the rest is up to him."
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.