McMahon was optioned to Triple-A Albuquerque on Thursday, Thomas Harding of MLB.com reports.
The Rockies need a fresh bullpen arm after sending out six relievers in Wednesday's 10-8 win over the Mets, so McMahon will swap places with right-hander Yency Almonte. The Rockies are set to face a string of left-handed pitchers, so the move will allow McMahon to see consistent at-bats with the Isotopes. He's hitting just .211/.283/.337 through 47 games with the big club this season.