Islanders center Mathew Barzal (lower body) and defenseman Zdeno Chara (upper body) each sustained an injury and did not play in the third period.
Phillip Danault scored at 3:43 of the first period to give the Kings a 1-0 lead. It was his 15th goal of the season.
"We were ready to go. We just didn't execute," Islanders captain Anders Lee said. "We didn't play to the level that they came out with. … The starts are something that we've focused on, and regardless of what we say, we've got to execute it."
Roy made it 2-0 at 7:51 on a sharp-angle one-timer below the left circle from Anze Kopitar. It was Roy's first goal of the season in his 51st game.
"I just saw an opportunity to jump in," Roy said. "Everyone was excited for me. It was a long wait."
Quinton Byfield
was credited with a goal 56 seconds into the second period after Islanders forward Cal Clutterbuck and defenseman Adam Pelech each made contract with the puck before it went between Varlamov's skates following a face-off.
Lee cut it to 3-1 at 9:47, but Brendan Lemieux scored at 19:31 to give Los Angeles a 4-1 lead.
"We got it stabilized in the second, and then, obviously, that goal at the end of the period, I thought that crushed us," Trotz said.
Jean-Gabriel Pageau scored on a backhand at 18:44 of the third period to make it 4-2.
Moore scored an empty-net goal at 19:43 for the 5-2 final.
Kings assistant Trent Yawney handled the coaching duties for Todd McLellan, who missed his second straight game after he entered NHL COVID-19 protocol Friday.
"It looked a lot like yesterday's game, where everybody had an impact on the game," Yawney said. "I mean, a little bit tougher back to back, but I thought the guys really showed some mental toughness."