The second-round draft choice was injured in Sunday's victory over the Tampa Bay Buccaneers while making a key play in the fourth quarter. The injury occurred as Harper helped tackle Tampa Bay tight end Alex Smith on a third-and-5 pass completion that came up short of the first down. The Bucs were forced to punt on the ensuing play, and Reggie Bush returned the kick 65 yards for the game-winning touchdown.
Tenth-year veteran Omar Stoutmire, who has appeared in 125 games and started 51 of them, will replace Harper in the starting lineup. Stoutmire, 32, went to training camp this summer as the projected starter but lost the No. 1 job to Harper during the preseason.
A former Alabama standout, Harper was the 43rd player chosen overall in this year's draft, and many were surprised the Saints chose him that high. But from the earliest minicamps, Harper, a bright and instinctive defender with a knack for getting to the ball, demonstrated solid abilities. While not particularly flashy, he plays with economy and efficiency and rarely makes mental errors.
"It's unfortunate because he's been playing well," Saints coach Sean Payton said.
The Saints placed Harper on injured reserve and re-signed 13-year
veteran Jay Bellamy to fill his spot on the roster.
Bellamy, who started all 67 games he played in for the Saints,
was released on Sept. 5. He had 451 tackles, 4½ sacks, nine
interceptions, six forced fumbles and five fumble recoveries.
Harper and second-year free safety Josh Bullocks provided the Saints one of the best young interior secondary tandems in the league. The Saints don't expect any complications after Harper undergoes surgery, which will occur when the swelling in the knee subsides, and he should be rehabilitated well in advance of next spring's offseason conditioning program.
He becomes the second New Orleans rookie, defensive end Rob Ninkovich being the other, to sustain a season-ending knee injury. And Harper is the first Saints starter who will miss a game this year. The upstart Saints host the Philadelphia Eagles on Sunday.
In five starts, Harper registered 33 tackles, with two games of seven tackles each, including Sunday's win over the Bucs. He also had a sack, a forced fumble and four passes defensed. His versatility was obvious in the team's Monday Night Football win over Atlanta on Sept. 25, as the Saints returned to the Superdome for the first time since Hurricane Katrina. Defensive coordinator Gary Gibbs used Harper on run blitzes in that game to help counter the Falcons' "spread option" attack.
Stoutmire had played with Dallas (1997-98), the New York Jets (1999), New York Giants (2000-04) and Washington (2005) before signing with the Saints as a free agent this spring. He has played in all five games this year, principally in dime coverage packages, and has one interception.
New Orleans officials made a concerted effort in the offseason to bolster the safety position and, with the loss of Harper, will now need to rely on that upgraded depth.
Senior writer Len Pasquarelli covers the NFL for ESPN.com. The Associated Press contributed to this report.