Paul Pierce Praises Celtics Rookie Grant Williams, 'That's a Great Pick!'

Paul Pierce loves everything about the Boston Celtics' top draft selection Grant Williams -- telling TMZ Sports straight-up," That's a great pick!"

Boston picked the forward from Tennessee with the 22nd pick in the 2019 NBA Draft -- and they're expecting big things from the 6'8", 236 pounder.

Besides being a stud in college, where he was a 1st team All American and a 2-time SEC Player of the Year ... he's also been praised for being super smart -- something that runs in his family.

In fact, Grant's mother, Teresa Johnson, is an engineer for NASA -- one of the first black, female engineers to ever do it ... amazing.

This is why Grant confidently shot down Kyrie Irving's flat-Earth theory in a video that's going viral -- where the NBA rookie tells it like it is ...

"Kyrie's 'Earth is flat' theory is scientifically not true," Grant said ... "That's just not true at all."

By the way, Kyrie is still technically on the Celtics -- so things could get weird in the locker room.

Then again, Kyrie is expected to leave when free agency begins in a few weeks ... so crisis averted?!