WATCH: [Blackhawks vs. Avalanche highlights]
"We don't play again until Thursday and the way we lost on Friday, to lose again today would have been tough," MacKinnon said. "To sit for four days, it would have been a big negative at the practice rink."
Captain Gabriel Landeskog held a brief pregame meeting to make sure everyone had the right mindset.
"It wasn't going to be like it was last year, here we go again," MacKinnon said. "Gabe relayed that to us. I don't know if anyone was thinking like that, but it's a brand new season, we have totally different players and we did a great job rebounding tonight."
The Blackhawks (5-5-2), who lost 2-1 to the Nashville Predators on Friday, have lost three games in a row and five of six.
"We've been playing good hockey at times, sporadically, but obviously it's not good enough," Blackhawks captain Jonathan Toews said. "We get ourselves down in games and it's just too little too late to try and come back. Throughout our games there's some good things here and there. We've just got to be way more consistent.
"We want to look at the big picture and stay positive. There's got to be urgency, but we can't overreact."
Rantanen scored two first-period goals and MacKinnon had a goal and an assist when the Avalanche took a 3-0 lead.