Muno, 26, hit .259 with a .372 OBP and .418 SLG last season for Las Vegas.
So far this season, Muno was hitting .235 with a .364 OBP in 22 plate appearances.
Montero, 24, had appeared in four games out of the bullpen for the Mets and posted a 4.15 ERA over 4 1/3 innings.
The team now has a seven-man bullpen and five-man bench.
Montero is talented. We've seen glimpses of it at the major league level.
Sending him to Triple-A will let him get a little more seasoning and build up his innings to come back to the team as a bona-fide starter.