Backup goaltender Philipp Grubauer was slated to start for Washington, but he played nearly two periods after Holtby was pulled from the loss to the Devils.
"I thought we were just kind of dead last game, myself included," Holtby said. "I think we all did what we needed to do throughout the day to get ourselves ready."
Colin Wilson and Tyler Jost scored for Colorado, which had won two in a row. Semyon Varlamov made 22 stops in the finale of a four-game trip.
"We just weren't as sharp as we needed to be tonight," Colorado coach Jared Bednar said. "We looked like we were dragging a little bit out there."
Connolly took a slick cross-ice pass from Madison Bowey and scored on a slap shot from the left circle 5:15 into the third. The Capitals were a man up thanks to a delayed penalty.
Niskanen made it 4-1 with his second of the season at 16:19. After Jost trimmed Washington's lead to two, Smith-Pelly put away Colorado with his fifth of the year with 45 seconds left.
The Capitals jumped in front on its first shot 5:25 into the game. Jakub Vranabeat Varlamov from the left circle for his ninth.
Wilson tied it 4:13 into the second period by flicking a backhander over Holtby after a quick pass from Matt Nieto behind the cage. Wilson nearly scored again later in the period but Holtby turned away his shot on a breakaway.
Trotz held out forward T.J. Oshie and said he also will miss Thursday's road game against Boston due to an upper-body injury. Trotz said he is hoping Oshie can return this weekend. ... The Capitals have scored first in 10 of their last 12 games. ... Bednar said forward Alex Kerfoot (foot injury) is day to day. He got hurt on Monday and was scratched for the matchup with Washington.