DENVER -- Second-base umpire Brian O'Nora came out of the Brewers-Rockies series opener Thursday night during the first inning with a back problem.

After Ryan Bruan's two-out double to center in the top of the first inning, first-base umpire and crew chief Ed Rapuano conferred with O'Nora, who remained in the game with some apparent discomfort until leaving in the middle of the inning.

Rockies assistant athletic trainer Scott Gehret walked O'Nora off the field and into the Rockies clubhouse.

Major League Baseball reported that O'Nora left the game with a "back issue." He will be re-evaluated over the next 24 hours and is listed as day to day.

The umpiring crew continued with three men through the middle of the fourth. In the bottom of the inning, Corey Blaser, a Minor League umpire who lives in Colorado, was in place at second, joining Rapuano, Ed Hickox behind the plate and Alfonso Marquez at third.