BOSTON — The assignment for Boston Celtics forward Marcus Morris in his first start this postseason was easy to explain but nearly impossible to execute.

His job: Guard LeBron James and keep him from running the Celtics out of their own gym in the Eastern Conference finals for the second straight year.

“He’s obviously the best player in the game,” said Morris, who during the week boasted 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 had 21 points and 10 rebounds while pestering James into a playoff-high seven turnovers – and a playoff-low 15 points – and Boston 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 the first quarter and never allowed the Cavs within single digits again. The Celts led by 28 when Cleveland Coach Tyronn Lue pulled James for good with 7:09 left.

Game 2 will be Tuesday night.

“I have zero level of concern at this stage,” said James, who was 5 of 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 Cavs missed their first 14 3-pointers of the game and shot 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 Gordon Hayward and Kyrie Irving on the bench in street clothes, the Celtics continued their stunning run 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 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.”

The Celtics took the floor to a 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, 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.

James 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.”

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.

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