Upshaw was playing for the Pistons' G-League team -- the Grand Rapids Drive -- when he collapsed on the court during the final minutes of a March 24, 2018 game.

The 26-year-old guard -- a former star at Hofstra -- never regained consciousness and died 2 days later from sudden cardiac death.

But Zeke's mom says he could have been saved if medical staff had rushed in and treated him immediately.

Instead, Jewel Upshaw says ... it took 4 crucial minutes before a doctor tended to Zeke -- no CPR, no chest compressions, no life-saving measures of any kind.

In fact, she claims the team doctor had "stepped out before the end of the basketball game and was not present nor available when Zeke collapsed."

Jewel claims when doctors finally tended to Zeke, the treatment was so poor -- she later learned Zeke's body "went without oxygen for another forty minutes ... leaving his brain completely oxygen deprived."

"At this point, there was no healthy brain left to save."

Jewel claims the NBA has had so much experience with players suffering sudden cardiac death over the years -- from Reggie Lewis to Conrad McRae -- there should be a proper protocol to handle these types of situations.

In her suit, Jewel says "there is simply no good reason for the defendants to have been unprepared for sudden cardiac death events such as Zeke's, and for their failure to react quickly to save a life."

"Had properly trained medical professionals administered expeditious and proper treatment, Zeke would still be alive."