Phillips was placed on the DL on July 11 after he had surgery to repair a torn ligament in his left thumb. He was batting sixth for Monday night's opener of a three-game set against the Cardinals.
Phillips is wearing a brace to prevent the thumb from bending backward but had two hits his last rehab game.
"I would not say he's 100 percent like brand new and that may not be until spring training of next year," manager Bryan Price said. "But he's definitely good to play."
Phillips' return comes at a crucial time for Cincinnati, which was swept in a doubleheader at Colorado on Sunday and is barely hanging on in the playoff race. The Reds also got third baseman Todd Frazier back after missing three games with lower back stiffness.
"Todd is probably not 100 percent either, but good enough to play," Price said. "These guys want to play, it says a lot about the character of our team."
The Reds activated reliever Logan Ondrusek from the DL and optioned right-hander Dylan Axelrod and catcher Tucker Barnhart to Triple-A Louisville. Right-hander Curtis Partch, who served as the 26th player for Sunday's doubleheader, was returned to Louisville.
The team has not determined a starter for Thursday night.