Eller moved up to the second line after Kuznetsov was injured and replaced him on the top power-play unit.
"He's a guy who is kind of our secret weapon," Washington captain Alex Ovechkin said.
Eller tied it 1-1 at 17:27 of the first from the bottom of the right circle off a pass from defenseman Michal Kempny. It was his sixth goal of the 2018 Stanley Cup Playoffs.
Then, from Kuznetsov's position as the low forward on the power play, Eller set up Ovechkin for a power-play goal from the bottom of the left circle to give the Capitals a 2-1 lead at 5:38 of the second.
Eller then made the pass to Orpik, who gave Washington a 3-1 lead on a wrist shot from the right circle at 9:41 of the second.
It was Orpik's first goal in 220 games, including 39 in the postseason, dating to Feb. 26, 2016. It was his first goal in the playoffs since April 21, 2014, for the Pittsburgh Penguins, a span of 55 games.
"I screamed, ears are still ringing," Capitals forward T.J. Oshie said. "He's one of those old-school pro guys. I've only played with three or four of them. He's one of those guys, and to see him get rewarded on the scoresheet, it's exciting."
Theodore scored on the power play to make it 3-2 at 17:47 of the second, but Holtby stopped the final 16 shots he faced.
"Thank God he's our goalie," Ovechkin said.
Goal of the game
Ovechkin's goal at 5:38 of the second period.
Save of the game
Holtby stopping Tuch at 18:01 of the third period.
Highlight of the game
Orpik's goal at 9:41 of the second period.
They said it
"We feel comfortable at any point in a series. That's one of the beautiful things of our group, but this is a tough place to win in Vegas and it's shown throughout the League. To come away with a split, it's good, but now we move forward. We need to take advantage of our own rink, our own familiarity, and focus on Game 3 because this game is gone now." -- Capitals goalie Braden Holtby
"We didn't play our best game, but I thought we had 39 shots and we still created a lot. It's one of those games where Holtby played real well, he was probably the difference in the hockey game."-- Golden Knights coach Gerard Gallant
Need to know
Ovechkin scored 116 goals, regular season and playoffs included, during Orpik's 220-game goal drought. … The Capitals were swept by the Detroit Red Wings in their only other Stanley Cup Final appearance, in 1998. ... The Golden Knights have scored the first goal in all nine of their home games this postseason, with each goal coming in the first period, including six in the first eight minutes.
What's next
Game 3 of Stanley Cup Final at Capital One Arena on Saturday (8 p.m. ET; NBCSN, CBC, SN, TVAS)