Walker was removed from Wednesday's game against the Twins with lower-back stiffness, Brian McTaggart of MLB.com reports.
Walker went 0-for-2 with a pair of strikeouts before being replaced by a pinch hitter in the sixth inning. He'll get an extra day to rest up before the Astros begin their next series Friday against the Rays, but if he's unable to play in the opener, Isaac Paredes would likely be next in line to start at first base, creating an opportunity at third for Brice Matthews.