The Heat announced the diagnosis after James' injury was evaluated Friday. He missed the final 5:50 of Thursday's game at Oklahoma City when he was struck inadvertently by defender Serge Ibaka on a drive to the basket.
James finished with a dunk but then crumpled to the floor, blood gushing from his nose. He left the court with a towel over his face.
He passed a concussion test but didn't talk to reporters after the Heat's victory, instead sitting in front of his locker with a towel draped over his head. The team was off Friday.
James has missed one game this season. The Heat won without him at Portland in December when he sat out because of a groin injury.
A model of durability during his 11-year career, James has never missed more than seven games in a season.
The Heat completed a 5-1 trip with their win at Oklahoma City. They also beat the Los Angeles Clippers, Phoenix, Golden State and Dallas. Sunday's game will be their first at home in 20 days.
Miami trails Eastern Conference leader Indiana by 1 1/2 games.