The 2001-02 league MVP with Montreal, Theodore evened his record at 2-2-1 with his best start of the season, losing a bid for his first Colorado shutout on Dany Heatley's goal midway through the third.
Theodore will return to the Bell Centre on Saturday night for his first game against the Canadiens since he was traded to the Avalanche for David Aebischer in a deadline deal on March 8.
"Obviously it's going to be special," Theodore said. "I was in that organization for 12 years. It was a great experience for me so it's going to be fun and exciting to go back to Montreal."
Colorado dominated the second period, outshooting Ottawa 20-7 after the teams traded scoring chances in the first.
"I thought that was our best period all year," Avalanche coach Joel Quenneville said.
Sakic scored his third goal of the season midway through the second to open the scoring. Richardson got his third with a short-handed effort with 3:16 left in the period to put the Avalanche up by two.
"Personally and as a team I'm really happy with the way the guys played," Theodore said. "You saw that as a team we bounced back right back after that first period. We played really hard and it's fun as a goalie to see that you know that your team is going to work hard in front of you."
Heatley, who had a team-record 50 goals last season, got his first of the season 9:45 into the third when he took a pass from Jason Spezza and fired a wrist shot that went in off Theodore's glove.
The Senators, who went 29-9-3 last season at Scotiabank Place, have lost each of their four home games.
"We wouldn't have guessed that was going to happen at the start of the year but we definitely haven't given ourselves as good a chance as we should to win games," Senators defenseman Wade Redden said. "Usually we're really good at this, especially at home. I don't know what it is. It just seems like we've got guys doing five different things sometimes all over the ice and until we get back to playing together, it's going to be the same thing."
Ottawa's Martin Gerber stopped 34 shots in his first start since a 4-3 loss to Buffalo on Oct. 7.
The Senators went 0-for-3 on the power play and have not scored in their last 31 attempts with the man advantage. Ottawa's power play is 1-for-33 overall in six games.
Colorado's power play also came up short, going 0-for-6. The Avalanche are 0-for-17 with the man advantage on the road.
Sakic opened the scoring 10:48 into the second, redirecting Andrew Brunette's pass from behind the net from the edge of the crease beyond Gerber despite being surrounded by Redden and his defense partner, Andrej Meszaros, and center Mike Fisher.
"We were running around there for what felt like 10 minutes in the second, just scrambling," Redden said. "Someone would get the puck and we'd just give it back to them, or we wouldn't get it out. They were working us down low and finally it broke. It was just a bang-bang play for Joe to put it in."
Richardson made it 2-0 during Ottawa's first power play opportunity when he beat Gerber with a shot into the top right corner.
- Theodore, who has 23 career shutouts, has not blanked an opponent since Feb. 28, 2004, against Carolina with Montreal, a span of 60 games.
- Senators D Joe Corvo made his season debut. Corvo, who signed as a free agent with Ottawa on July 1 after he had 14 goals and 26 assists with Los Angeles last season, missed Ottawa's first five games because of a broken foot.
- Rick Middleton and Gary Leeman are tied for the longest scoring drought from the start of a season by a player who scored at least 50 goals the previous season, according to the Elias Sports Bureau. Middleton didn't score through his first seven games of 1982-83, and Leeman matched that in 1990-91.