The worker was bitten and clawed as he used pepper spray to protect himself from the lion. Officials said the man's injuries were not life-threatening.
The attack occurred as the worker moved the big cat from an outdoor viewing area to an indoor habitat after the zoo had closed, WDIV-TV, Detroit reported.
"This happened at a time when the park was not open to visitors. The lion never left the secure area inside the building, so there was never any risk to visitors," zoo spokesman Scott Carter said.
The worker "was conscious and reported what happened as he was being treated," Carter said.
Carter said the attack, on Saturday, was a "very rare occurrence."