The Florida South Sentinel reported that the 6-foot-7, 230-pound Marion was diagnosed with plantar fasciitis in his left foot and heel.
The injury, which affects ligaments, causes weakness and swelling.
Marion was already bothered by back spasms that forced him to miss 11 of the past 12 games.
"The back is a problem," Marion told the newspaper, "but I've had trouble with my heel and foot for years."
The Heat acquired Marion from Phoenix for Shaquille O'Neal in a Feb. 6 trade.
Other Heat players out for the season include Dwyane Wade (knee), Udonis Haslem (ankle), Alonzo Mourning (knee) and Dorell Wright (knee).
"I don't even think about it anymore," Coach Pat Riley said. "We're just moving on and finishing the season."