Sources told ESPN and ESPN.com that the former Heisman Trophy winner is likely to miss the rest of the season.
ESPN's Chris Mortensen first reported Friday afternoon that Bush had a partial tear of the posterior cruciate ligament, and Saints officials subsequently confirmed the injury.
League sources told ESPN.com that Bush sustained the injury in a Nov. 4 game against the Jacksonville Jaguars, that he aggravated it in another game and has played the past month with it.
"It just got to the point, though, where he couldn't [play] anymore," a league source said. "He's a tougher kid than people realize, but it was only getting worse. At this point, he's just really beaten up."
The former Southern California star and the second player chosen overall in the 2006 draft, Bush has already undergone at least one MRI examination and will visit noted orthopedic surgeon Dr. James Andrews in Birmingham, Ala., on Monday. A determination on how best to treat the injury will then be made.
It isn't unusual for a PCL injuries not to require surgery, and such tears typically heal with rest and rehabilitation.
Bush practiced on Thursday and did not indicate he was having any trouble when he spoke to reporters after practice. However, he became sore that evening and the team ordered a precautionary MRI, which indicated a tear in his posterior cruciate ligament.
The injury is the latest setback for Bush, who acknowledged on Thursday that he has performed below his expectations since taking over as the Saints' featured running back after Deuce McAllister's season-ending knee injury in Week 3.
In 12 games, Bush has rushed for 581 yards and four touchdowns on 157 carries. He also has 73 receptions for 417 yards and two touchdowns. But his longest plays from scrimmage have been a 22-yard run and a 25-yard reception, and the usually sure-handed Bush has 10 dropped passes and has fumbled eight times, losing three.
He has appeared in 28 games in his two seasons in the league and has 1,146 rushing yards and 1,159 receiving yards, with 14 touchdowns.
Len Pasquarelli is a senior NFL writer for ESPN.com. Information from ESPN reporter Chris Mortensen and The Associated Press contributed to this report.