Lukas Dostal made 41 saves for the Ducks (12-20-0), who won their previous two games.
"I think it was the worst game we played all year in terms of execution," Anaheim coach Greg Cronin said. "We couldn't shoot a puck, it was springing off our sticks, we had shots in the slot we whiffed on. ... It was a comedy of errors."
Ducks rookie forward Leo Carlsson, the No. 2 pick in the 2023 NHL Draft, was injured at 9:24 of the third period when Flames defenseman MacKenzie Weegar fell on his leg. Carlsson could not put on any weight on his right leg as he was helped off the ice.
"You can see when Weegar fell on him, any sport when that happens, it’s not good," Cronin said.
Calgary took a 1-0 lead at 2:24 of the first period. Connor Zary had the puck along the wall in the Anaheim zone and passed it to DeSimone, who scored a one-timer from the high slot.
"I've seen [Zary] pull up like that a couple hundred times, so I knew he was going to get it to me somehow," DeSimone said. "I just had to be ready for it."