"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."
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, follow him on Twitter @m_sheldon and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.