The Saints discovered the condition Thursday during a follow-up physical, coach Sean Payton told reporters Friday.
The trade from the Eagles and subsequent physical was a life-saver by the Saints' physician, Dr. John Amoss.
"This is a pretty serious condition and not only will it require open-heart surgery, it's going to require it pretty quickly," Payton said. "Dr. Amoss is the one that discovered it and basically saved his life with his findings.
"So, he did further tests and evaluations and we met twice yesterday. Certainly, news like this is a big alert for him."
Dorenbos, 37, will be placed on the non-football injury list, ending his season.
The Saints traded a 2019 seventh-round pick for Dorenbos on Aug. 28. The current situation could rescind the transaction.
"That will handle itself," Payton said. "In other words, this is a pre-existing condition and this is something that genetically he had at birth."
Dorenbos becomes the second Saints player to deal with a heart condition. Defensive tackle Nick Fairley landed on the NFI list in June after the discovery of an enlarged heart.
Dorenbos was set to enter his 16th season. He spent the past 11 years with the Eagles after starting his career with the Buffalo Bills and Tennessee Titans. With the Eagles, he appeared in a franchise-record 162 consecutive regular-season games and earned two Pro Bowl berths.
"I can't speak for Philadelphia or the other teams he's been with, but I do know that that's something that came up on our physical," Payton said. "There's a blessing to it in some regards regarding him.
"Finding an aneurysm this size, it's something that will require immediate surgery and credit Dr. Amoss. He was the first one that kind of caught the red flag."
The popular player among teammates and fans is also known for his talents as a magician, making television appearances and becoming a finalist on the NBC show "America's Got Talent" in 2016. Dorenbos turned to magic in his childhood as a way to deal with a family tragedy when his father murdered his mother when he was 12.
Payton said the Saints plan to work out long snappers ahead of Monday night's season opener against the Minnesota Vikings. Former Denver Broncos long snapper Jeff Overbaugh will be among the group of players working out Friday, according to the New Orleans Times-Picayune.