Reynolds was a late scratch from the lineup for Wednesday's game versus the Cardinals due to back spasms, Alex Stumpf of MLB.com reports.
Reynolds will be examined and treated by the Pirates medical staff and is considered day-to-day for the time being. It's the first time all season that he's been out of the lineup. The Pirates have a scheduled off day, so Reynolds will be afforded some extra rest before their next game Friday in Arizona.