Varlamov replaced Aittokallio, who gave up three goals on 18 shots, to start the third period.
"It's not easy for him, and I certainly understand that," Roy said. "But at the same time, I thought we had some momentum and it was the call to make at that moment. If I had a crystal ball, yes, I would have kept him there."
Aittokallio, recalled from Lake Erie of the AHL Friday, made his first career appearance in a start last April 11 at Los Angeles, a game in which he left in the third period due to leg cramps.
Johnson opened the scoring at 7:15 of the second period, Barberio made it 2-0 about 1 1/2 minutes later when his shot from the top of the right circle deflected off a Colorado defender and past Aittokallio. Filppula completed Tampa Bay's three-goal period with just under 2 minutes to go.
"He played a good game," Roy said of Johnson. "That's all I'm going to say."
Mitchell got the Avalanche on the scoreboard when his redirection eluded Ben Bishop with 1:27 left in the second and Barrie scored 30 seconds later to pull Colorado within one.
"We gave ourselves a real opportunity," Mitchell said. "We ran out of time in the second period. If we had a little bit more time on the clock, we had all the momentum going our way. Give credit to them, they came out and played a solid third period."
MacKinnon had one shot, won 55 percent of his faceoffs, and was plus-1.
"I can't believe he's 18," Cooper said. "Pretty impresive kid."
St. Louis has 964 points, which is one away from tying Maurice Richard for 85th all-time. He is one assist away from becoming the 77th NHL player to reach 600. ... This was the first game in Tampa between the teams since Dec. 4, 2010. ... Roy never won a road game as a goalie against Tampa Bay, going 0-7-1, during his Hall of Fame career. Roy, the first-year Colorado coach, is second on the NHL goalie victories list with 551. ... Lightning star Steven Stamkos did light contact drills for the first time during the morning skate since breaking his right shin last November. ... Colorado LW Alex Tanguay, who returned Friday after missing 36 games with knee and hip injuries, had a scheduled night off.
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TAMPA, FL - JANUARY 25: Martin St. Louis #26 of the Tampa Bay Lightning leads his team to the ice for pre-game...