Brock Nelson and Victor Olofsson also scored, and Scott Wedgewood made 29 saves for the Avalanche (38-9-9), who have won three of past four games.
Nathan MacKinnon was not in the lineup for Colorado after playing in Team Canada’s 2-1 overtime loss to Team USA in the gold medal game of the Olympic Winter Games Milano Cortina 2026 on Sunday.
Dylan Guenther scored twice, and Karel Vejmelka made 21 saves for the Mammoth (30-24-4), who had won two in a row.
Logan Cooley had an assist in his return to the lineup for Utah after missing the previous 28 games with a lower-body injury.
Nelson appeared to score for the Avalanche at 8:37 of the first period after tipping in a shot from Brent Burns, but the play was overturned after Utah successfully challenged for goaltender interference.
Kelley gave Colorado a 1-0 lead at 3:26 of the second period, firing a wrist shot past the blocker of Vejmelka.
Olofsson made it 2-0 at 9:13, beating Vejmelka with a wrist shot off a pass from Sam Malinski.
Guenther’s power-play goal cut it to 2-1 at 10:49, sending a one-timer past the blocker of Wedgewood off a pass from Mikhail Sergachev.
Nelson extended it to 3-1 at 12:20, scoring past Vejmelka with a one-timer off a pass from Gabriel Landeskog.
Guenther scored his second goal at 13:20 to cut it to 3-2 when he found a loose puck in the slot and beat Wedgewood blocker side.
Necas scored on the power play to make it 4-2 at 17:17 to make it 4-2, finishing glove side with a one-timer off a feed from Cale Makar.
Brett Kulak played 19:03 of ice time in his Avalanche debut. The defenseman was acquired from the Pittsburgh Penguins for defenseman Samuel Girard and a second-round pick in the 2028 NHL Draft on Tuesday.
