Downs, the former Giant, was warming up in the bullpen in the eighth inning Wednesday night and would have come in to pitch had reliever Xavier Cedeno gotten his pitch count past 45. Cedeno threw 40 pitches over three scoreless innings.
Astros outfielder Brian Bogusevic, who was drafted as a pitcher before being moved to the outfield four years ago, pitched an inning June 7 against the Cardinals in his first action on the mound in the Major Leagues.
"I didn't want to bring Downs in like that, but at the same time you had to start protecting the bullpen," manager Brad Mills said.
The versatile Downs, who started at first base Thursday and homered in his first at-bat, can play just about any position on the field and is Mills' emergency catcher. He even caught briefly in the bullpen this spring to get familiar with the role.