McMahon, 21, is batting .233/.331/.325 over 144 plate appearances at Double-A Hartford this season.
Power has been McMahon's calling card through his previous three stops in the minors, but he's shown precious little in that regard since moving to Double-A this season. He has yet to hit a home run, and his slugging percentage is down 150 points compared to what he posted at High-A Modesto in 2015. McMahon has at least gone a perfect 7-for-7 on stolen-base attempts, but he'll need to start cranking out extra-base hits in order for the Rockies to feel more positive about his development.