Stewart was placed on the 15-day DL on Monday with left knee discomfort.
The Pirates had been anticipating this move, as Stewart had been held out of the starting lineup on back-to-back days. The team also went ahead and claimed Eric Fryer off waivers from St. Louis, who will be activated Monday in a corresponding move. As long as the ailment doesn't turn out to be too serious, Stewart should be back with the Pirates shortly after the All-Star break when he's first eligible.