Harang, 35, went 5-11 with a 5.76 ERA in 22 starts for the Mariners this season, striking out 87 and walking 28 over 120 1/3 innings. He surrendered seven runs each in three of his final five starts, and was released Sunday after previously being designated for assignment.
Harang has pitched for five teams over 12 big league seasons, going 110-115 with a 4.29 ERA.
Las Vegas is scheduled to open the Pacific Coast League playoffs on Friday, and could use some rotation depth for the tournament. Not only are right-handers Rafael Montero and Jacob deGrom both approaching innings limits, but deGrom left his last start after two innings with right shoulder stiffness.
The Mets, meanwhile, are using Daisuke Matsuzaka and Carlos Torres as their fourth and fifth starters, but Matsuzaka struggled in his first two outings and Torres is capable of working out of the bullpen.
Most importantly from an organizational standpoint, the Mets are trying to limit rookie Zack Wheeler's innings down the stretch. Wheeler has already thrown a career-high 151 2/3 innings over two levels this season, and is tentatively scheduled to pitch five more times.
"If he's spaced out and we keep track of him, he should be able to get to where we'd like him to without a problem," Collins said. "But he's pitching pretty good."
Anthony DiComo is a reporter for MLB.com. Follow him on Twitter @AnthonyDicomo. This story was not subject to the approval of Major League Baseball or its clubs.