Praises Celtics Rookie Grant Williams

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.

Which is why Grant confidently shot down Kyrie Irving's flat-Earth theory in a…