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 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 tonight.

“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.

(SD-Agencies)