"We're trying some different orthotics to stabilize that foot," Reds head trainer Paul Lessard said.
Phillips is considered day to day, according to manager Bryan Price.
"My understanding is it happened [on Monday], when he stole second base before [Kristopher] Negron came up, and then there were some lingering effects from that that we didn't know about until he made mention of them during the game," Price said. "Then it became very apparent it was getting worse, not better."
Phillips, Votto team up for outCozart missed the last two games, and five of the previous eight, with wrist soreness after he was hit by a pitch on May 3.