CLEVELAND — Paul Pierce didn’t mince words. He doesn’t have to. He’s been given good cover.

“I think he’s going to be one of the great players in Celtic history,” Pierce said of Jayson Tatum.

Jayson Tatum. Rookie Jayson Tatum. Just stopped being a teenager in March Jayson Tatum.

Paul Pierce is one of the great players in Celtic history — his number hangs above the parquet floor in grand evidence — but how many people were anointing him so as he finished his rookie year? There was every fair prognosis that Pierce was going to be really good, a star even. But didn’t have the season that Tatum did.

Tatum’s gone from 13.9 to 18.4 points a game in the postseason and has been averaging 20.5 since Game 6 in Milwaukee. Pierce averaged 16.5 points as a rookie, but he didn’t get to participate in the playoffs until his fourth season.

“He’s already off to such a great start,” said Pierce. “He’s mature beyond his years. He doesn’t look rattled by the moment. Sometimes you see rookies have problems when they get to this point. I saw Ben Simmons against the Celtics, and it looked like the moment was too big for him. He was wide-eyed out there. He looked confused.

“I don’t see that in Jayson Tatum. And if he didn’t have a team full of guys who are really good players and with two All-Stars, he could easily have won Rookie of the Year. I think he’s just starting to see his potential. He’s just 20 years old and he has a long way to go, but he’s just scratching the surface of how great he can really be.”

Tatum is scratching pretty hard in this postseason, hard enough to leave a mark. The latest scratch — 24 points, four rebounds and four steals in Game 5, plus the assertiveness and utter lack of hesitation — has moved the basketball world to the edge of its seat.

Pierce, too.

Yesterday morning, he was moved to text.

“He doesn’t know that he’s the guy yet,” Pierce wrote. “When he realizes that, watch out.”

Will the #Celtics steal a win — and the series — in Cleveland tonight against the @cavs ? #NBA — Boston Herald (@bostonherald) May 25, 2018

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He hit a familiar theme in another message, “Mature beyond his years. Scary.”

Back in the earlier conversation, Pierce expanded on his big-game commentary.

“He’s an ultra talent, but some players are made for the regular season and some are built for the playoffs,” said Pierce. “And Tatum is obviously built for the playoffs. He’s not going to have the stats that these other guys like Donovan Mitchell have and Simmons, but you can tell he was made for the playoffs. He thrives in the playoffs.”

There was another player in recent Celtic history that did, too.

“Oh, I see a lot of myself in him,” Pierce said. “It just seems like he’s outsmarting his opponent all the time. He’s constantly thinking, how can he get his shot off? He can get his shot any time he wants, he’s great in that mid-range area, he’s very deceptive, and he’s showing me a lot.

“The way he already uses his body, once he gets stronger, he’s going to be unstoppable. He’ll be getting to cup, finishing and getting and-one’s. When he’s not blowing by his man, he’s using his body to create space. I’m seeing those little things that he’s doing, his footwork, you know, the half-spin he made in that game against Philly. I’m like, wow. He’s got a great feel for the game. He doesn’t look like a rookie at all.”

Among the Tatum amazements for Pierce is that the newer Celt is doing all this after just one year of college, a season in which he was hampered by an early foot injury.

Pierce spent three years at Kansas, increasing his scoring average from 11.9 to 16.3 to 20.4.

“I had a chance to develop, get stronger,” Pierce said. “He’s two years removed from high school and he’s playing like this. I mean, that’s the scary part. That’s why he’s just scratching his potential.

“He’s only going to get better as he gets smarter, gets in the gym and figures out other ways to get things done. He’s showing he can do more. And maybe it’s a blessing that Kyrie (Irving) isn’t out there right now, because he’s seeing that he can handle the ball. He can come off pick-and-roll situations, where you couldn’t do that before because you had Kyrie being pretty ball-dominant out there.”

For now, Pierce has nothing to teach Tatum. Key words: “for” and “now.”

“No, I don’t want to mess up his run, man,” Pierce said. “He’s fine. I would love to work with him, though, this summer. I’ll probably call Danny (Ainge) and see if that’s possible, but I would love to work with him. I see some things in his game where I know I could help.”

One of the great players in Celtic history helping a kid accelerate the process to becoming another.

Red Auerbach just smiled and lit another cigar.