Primarily guarded by Marcus Morris, LeBron James finished with 15 points, nine assists and seven turnovers in Cleveland’s Game 1 loss. (Michael Dwyer/AP)

Ahead of the Eastern Conference finals, Boston Celtics forward Marcus Morris huddled for hours inside his man cave like a student cramming for a big test.

“Defending LeBron James 101” has become a master class, and already the Indiana Pacers and Toronto Raptors have flunked out of the NBA playoffs because they could not find a successful and consistent plan against the Cleveland Cavaliers’ superstar.

Morris waited next in the long line of LeBron defenders, and so he studied the tendencies of the ones who came before. He watched plays on Synergy. He downloaded clips on YouTube. He projected games on his flat screen, pressing the rewind button once, twice, then three times just to figure out what, if anything, he could take from the previous two playoff teams that had been vanquished by the King.

Morris sought inspiration, but he stumbled upon a harsh truth.

“Them dudes can’t guard. That’s what I did pick up,” Morris said, bluntly.

On Sunday afternoon, as the Celtics rolled to a 108-83 win in Game 1 at TD Garden, Morris discovered yet another truth.

The answer to the most confounding question of this generation — How do you stop the greatest player in the game? — cannot be solved by one studious defender. The obsession in slowing down James, who finished with 15 points on 5-for-16 shooting Sunday with nine assists but also seven turnovers, must be shared.

“It’s a team effort,” Morris said after earning his first career playoff double-double with 21 points and 10 rebounds while playing as the primary James stopper. “It’s not just me. Everybody played their part in guarding him. He’s obviously the best player in the game, and you need multiple guys and a team to guard him an entire game. I just think we did a great job of that.”

Boston’s defense looked like a buddy system. It was Morris, who was promoted to the starting lineup for the first time this postseason, getting away with subtle shoves during moments in Cleveland’s offensive plays when James didn’t possess the ball, or the officials’ attention.

Then Jaylen Brown knew exactly what to do when Morris picked up two early fouls and defending James became Brown’s responsibility.

Terry Rozier, a tough kid nicknamed “Scary Terry” but a smaller guard nonetheless, did not shy away from switches when the 6-foot-8, 250-pound James threw the force of his body to post him up.

In the first quarter, James faced five different defenders who all refused to give away anything easy during the Celtics’ breakout 28-4 run.

“Coach [Brad Stevens] was talking about making him feel us,” said Boston reserve forward Semi Ojeleye, one of the many defenders who picked up James for a play or two. “Trying to make him uncomfortable. I think when he’s comfortable, he picks guys apart. That’s the way he wants. It’s a tough matchup, so we just tried to take him out of rhythm.”

While teammates provided backup, Morris willingly filled James’s dance card for much of the one-on-one action. Although Morris picked up three fouls in the first half, he did not turn tentative and still defended James with gusto on 24 plays. In those moments, Morris held James to 2-for-6 shooting and five points, according to ESPN Stats & Info.

“Because I’m a competitor,” Morris responded when asked why he had looked forward to the matchup.

Confidence streams through the Morris family bloodline. Back in November, his twin brother, Markieff, who plays for the Washington Wizards, answered “Hell yeah” when asked whether he could have helped stop James from scoring 57 points against the Wizards if he had been healthy. Marcus, similarly, wanted to test his will against James and started preparing as soon as the conference final matchup was set.

Although it’s not uncommon for players to spend off-the-clock hours with their personal trainers, Morris brought his guy from his hometown of Philadelphia to Celtics practices held in Waltham, Mass. After the team sessions, Chuck Ellis said the pair headed to Morris’s suburban home, then straight to the room with the big television.

“A lot of film. That’s what our days consist of outside of the gym,” Ellis said. “We’re at his house watching film.”

Although Morris didn’t learn a single thing from the Pacers and Raptors, Ellis said they also pored over clips featuring Kawhi Leonard and Andre Iguodala. Those players had previously created highlights in defending James, and Morris learned some essential dos and don’ts.

By the time Morris dropped low into his first defensive stance Sunday, he was prepared.

“Our confidence level is very high,” Morris said. “At the end of the day, all the talking is done off the court. Once we step between them lines, we have to compete no matter who we have out there, and that’s what we’ve been doing.”