Kemp broke a rib while crashing into the outfield wall during Sunday's loss to the San Diego Padres, according to John Fay of the Cincinnati Enquirer. He'd been originally diagnosed with a chest wall contusion.
After being injured, the three-time All-Star initially stayed in the game and struck out in the next half-inning before being replaced defensively by Scott Schebler.
Kemp is off to a brutal start in his first year with the Reds following a blockbuster offseason trade that sent him, fellow outfielder Yasiel Puig, and left-handed pitcher Alex Wood from the Los Angeles Dodgers to the Midwest.
In 20 games with his new team, Kemp is hitting just .200/.210/.283 with one home run and five RBIs. The 34-year-old is making $21.5 million in 2019 and is in the final season of a massive eight-year contract he signed with the Dodgers in 2011.
In a corresponding move on Tuesday, Phillip Ervin was promoted from Triple-A Louisville to take Kemp's spot on the Reds' 25-man roster.