Zeke Upshaw, a forward on the Grand Rapids Drive team, suddenly dropped to the court on his stomach during a game against the Long Island Nets.
The Drive said in a news release Sunday that further updates on Upshaw's condition will be provided when available.
MLive reported that Upshaw was taken from the arena to a nearby hospital.
Upshaw's mother, Jewel Upshaw, told WOOD TV8 in Grand Rapids: "The next 24 hours are critical".
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I come to win. "LA is a place for stars, and he's going to be one of the brightest stars that's ever played in this market". I'm not there for something else.
"I immediately knew it was awful because he wasn't moving", Drive public address announcer Eric Zane told MLive. The broadcast did not originally show what happened to Upshaw, but it was described as a sudden collapse that sent trainers and doctors from both teams in Upshaw's side.
It will be the first time the franchise makes an appearance in the G League playoffs.
Upshaw is in his second season with the Drive.
Upshaw, 26, played three years at Illinois State before finishing his college career at Hofstra in 2013-14. More than a quarter of National Basketball Association players come from the developmental league.