Cops found a credit card holder they suspected was used to conceal cocaine during Robert Nkemdiche's arrest earlier this month ... and they asked the NFL stud straight-up, "What's the white stuff in there?"
It's all in police video obtained by TMZ Sports ... which shows a cop pulling over Nkemdiche on June 6 in Arizona for speeding -- and then finding the Cardinals DL had a warrant out for his arrest stemming from a previous traffic violation.
When the cop gets Robert out of the car to cuff him ... he realizes the 6-foot-4, 314-pounder's wrists are too big for handcuffs .