His job: Guard LeBron James, and keep the four-time NBA MVP from running the Celtics out of their own gym in the Eastern Conference finals for the second year in a row.

“He’s obviously the best player in the game,” said Morris, who during the week boasted that he was up to the challenge and on Sunday explained why he wanted it.

“Because I’m a competitor. He’s the best player, and I’m going to be able to tell my kids this one day.”

Morris scored 21 points and added 10 rebounds while pestering James into a playoff-high seven turnovers — and a playoff-low 15 points — and the Celtics opened a 21-point, first-quarter lead to scorch Cleveland 108-83 in Game 1.

Jaylen Brown scored 23 points and Al Horford had 20 for Boston, which ran off 17 points in a row in the first and never allowed the Cavaliers within single digits again. The Celtics led by 28 when Cleveland coach Tyronn Lue pulled James for good with 7:09 left.

Game 2 is Tuesday night.

“I have zero level of concern at this stage,” said James, who was 5 for 16 from the floor and missed all five 3-point attempts.

“I’ve been down before in the postseason, but for me there’s never any level of concern — no matter how bad I played tonight, with seven turnovers, how inefficient I was shooting the ball,” he said. “We have another opportunity to be better as a ball club coming in Tuesday night, and we’ll see what happens.”

Kevin Love had 17 points and eight rebounds, and James added nine assists and seven boards. The Cavaliers missed their first 14 3-point attempts of the game and shot just 32 percent in the first half.

By that time, Boston led 61-35 — the biggest halftime playoff deficit in James’ career.

“I think we’re very alert to the fact that we’ll get a heavyweight punch on Tuesday night,” Celtics coach Brad Stevens said. “It’s another great challenge, another great opportunity to experience something for this team.”

With injured stars Gordon Hayward and Kyrie Irving on the bench in street clothes, the Celtics continued their stunning run this season with what may have been the most surprising turn of events yet. A year after dropping the first two games at home against Cleveland in a five-game East final, the Celtics pounced on James, and the favored Cavaliers were never really in it.

“The last couple of playoffs, and our meeting at the end of year, they blew us out of the water,” Boston guard Marcus Smart said. “We’ve got a different team, just like they do, and a lot of younger guys. So for them to see that, and have that feeling like we did tonight, is huge.”

TIP-INS

Cavaliers: Kyle Korver’s 3-pointer with nine minutes left in the third period was their first after missing 14 in a row. It cut the deficit from 28 points to 65-40. … Tristan Thompson had eight points and 11 rebounds.

Celtics: The Celtics improved to 8-0 at home this postseason. They do not have to win on the road to reach the NBA Finals. … Boston’s 36-18 lead at the end of one quarter was the second-largest in a playoff game in franchise history.

GET HYPED

The Celtics took the floor to a hype video that began with the broadcast of Hayward’s injury in the first quarter of the first game, at Cleveland. A variety of commentators predicted the team’s demise, including Hall of Famer Charles Barkley saying: “Their season’s over.”

As more players went down to injury — including Irving, who came over from the Cavaliers last summer but was done for the season in March — the prophesies grew even gloomier.

But there the Celtics were, back in the Eastern Conference finals against the Cavaliers for the second year in a row — with Hayward, Irving, Daniel Theis and Shane Larkin all injured and Stevens down to an eight-man rotation.

It was more than they needed.

Starting in place of Aron Baynes, Morris backed up his boast that he could cover James better than anyone except Kawhi Leonard. The Cavaliers star had seven of Cleveland’s nine turnovers after accruing eight in the entire four-game, second-round sweep of the Raptors.

“Our confidence level is very high,” said Morris, who recorded the first postseason double-double of his career. “The younger guys to the older guys, we think we can compete and play with anybody. At the end of the day, all the talking is done off the court.”

FAST START

Horford made his first seven shots and scored 10, including eight straight, during the 17-0 run that turned a three-point deficit into a 21-7 lead. After James wiggled his way to a layup — Cleveland’s first points in 4 minutes, 43 seconds — Boston ran off eight more points in a row; Brown had six of them, and he finished the quarter with 13 points and five rebounds.

Cleveland scored seven straight points early in the third and finished the quarter with six in a row to make it 78-64. But Boston made the first three baskets in the fourth and, after running off nine in a row to make it 96-68 with 7:09 left, the Cavs conceded.

UP NEXT

Game 2: Tuesday night, Boston.