McMahon has been called up from Triple-A Albuquerque and will start at first base Sunday against the Athletics, Patrick Saunders of The Denver Post reports.
McMahon will step in at first base to relieve Ian Desmond, who hasn't had a true day off since June 17. The 23-year-old McMahon owns a .877 OPS at Albuquerque this season, but that hasn't translated to the majors, where he's slashing .211/.283/.337 across 106 plate appearances. He'll look to get back on track in the final months of the season, though it appears unlikely he has a path to consistent playing time.