Allen Iverson is offering up NBA advice to Cooper Flagg ... and as a former No. 1 overall pick who went on to become a Hall of Famer -- the Dallas Mavericks star sure would be wise to listen up!!

The Answer doled out several tips to this year's top draft selection while chatting with TMZ Sports just before Flagg's Thursday night debut. Getty

Iverson's first bit of guidance was straightforward ... as he implored Flagg to take things "one day at a time."

Then, the former point guard -- who just premiered his new docuseries "ALLEN IV3RSON" in NYC -- told Flagg effort is the biggest thing to take care of when he hits professional hardwoods.

"The most important thing, and I think this will take care of everything, is playing every game like it's your last, you know?" Iverson sa

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