The Cleveland Cavaliers forward was 5 for 16 for 15 points and turned the ball over seven times in a 108-83 loss to the Celtics Sunday afternoon.

Jim Fenton The Enterprise @JFenton_ent

BOSTON – He averaged 34.3 points on 55 percent shooting in the first 11 Cleveland Cavaliers playoff games this spring.

That came after LeBron James had scored 27.5 points on 54 percent shooting in 82 regular-season games.

But in the opener of the Eastern Conference finals Sunday afternoon, James had a rare off-day when he was limited to 15 points on 5-for-16 shooting, missed all five 3-pointers and turned the ball over seven times in 36:07.

Without James being James, the Cavaliers had no chance against the Celtics and were handed a 108-83 loss at the TD Garden.

“I’ve seen every coverage, pretty much,’’ said James, who looked frustrated at time as the Celtics dominated the Cavaliers. “For me, Game 1 has always been a feel-out game for me, if you’ve followed my history. So I’ve got a good sense of the way they played me today and how I’ll play going into Game 2 (Tuesday night).’’

The Celtics put Marcus Morris in the starting lineup to guard James, and he had help throughout. James had all but two of the Cavaliers’ turnovers and was even called for a rare travel.

“I thought they had a great game plan Game 1,’’ said James. “(Morris) was the start of it. He was my matchup and I think they did a great job of communicating throughout the whole game, knowing where I was and knowing where our teammates were.’’

James was a minus-32 for the game, which according to stats guru Dick Lipe, matches the worst performance of his playoff career. James was also minus-32 in the NBA Finals against the San Antonio Spurs on June 11, 2013.

James has won seven straight Eastern Conference titles with the Cavaliers and the Miami Heat, and he was never behind, 1-0, in the conference finals during that stretch until now.

“I have zero level of concern at this stage,’’ said James. “I didn’t go to college, so it’s not March Madness (one and done). You get better throughout the series. You see ways you can get better throughout the series.

“But I’ve been down, 0-1, I’ve been down, 0-2. I’ve been down before in the postseason. There’s never no level of concern, no matter how bad I played tonight with seven turnovers, how inefficient I was shooting the ball.

“I’m just as confident going into a series whether it’s a 0-0 series or if I’m down, 0-1. We have another opportunity to be better as a ballclub coming in Tuesday night and we’ll see what happens.’’

Plenty of misses: The Cavaliers missed their first 14 shots from 3-point range and finished 4 for 26. They had connected on 35 percent during the first two series.

“Three-point shots (are) a part of our DNA,’’ said James. “It’s what makes us the best team that we can be. We’re going to take those same looks going into Game 2 if the opportunity presents itself.’’

Said coach Tyronn Lue: “We had some great shots early we didn’t make, and it just kind of snowballed from there. That’s not us, but they did a good job of closing out.’’

Around the rim: The Celtics are now 8-0 in the postseason at the Garden. The Celtics ended the 2017 playoffs with three straight home losses to the Cavs. … Injured Celtics Kyrie Irving, Gordon Hayward, Daniel Theis and Shane Larkin were on the team’s bench. … Morris picked up two fouls in less than three minutes to open the game, but coach Brad Stevens stayed with him. “Ninety-nine percent of the time I lean on error of the side of not taking them out,’’ said Stevens. “If he fouls out in the first quarter, then somebody else has to play. That’s the way it goes.’’ … Jayson Tatum, who had scored at least 20 points in seven straight playoff games, had 16 points. … The Celtics starters outscored their counterparts, 88-46, with everyone but Terry Rozier (eight points) hitting double figures. … Former Celtics forward Jeff Green had six points and one rebound in 20 minutes while ex-Celtics center Kendrick Perkins was on the Cavs inactive list. … The rest of the games in the series will begin at 8:30 p.m.