It defied explanation: The Giants led the Tennessee Titans 21–14 with less than three minutes left in Sunday’s game. Tennessee faced fourth-and-10. As Tennessee quarterback Vince Young dropped back to pass, Giants rookie defensive end Matthias Kiwanuka rushed into the pocket and grabbed him. If Kiwanuka had tackled Young, the game would already be fading from memory. Instead of driving him into the ground, Kiwanuka let go, and Young took off running. He picked up 19 yards on the play and finished that drive with a game-tying touchdown pass.

Minutes later, a Tennessee field goal clinched the 24–21 final score of a game the Giants led 21–0 with 10 minutes left. Kiwanuka was the goat of the Giants’ worst collapse since a 38–14 lead turned into a 39–38 loss to the San Francisco 49ers in the 2002 playoffs. Sports Illustrated writer Peter King labeled Kiwanuka’s blunder “one of the dumbest and most inexcusable plays in NFL history — and one which will take its place alongside the Joe Pisarcik fumble in 1978 in dubious Giants’ lore.”

But it’s the way the NFL instructs its referees to pamper quarterbacks that is dumb and inexcusable. As Kiwanuka explained after the game, he let go of Young because he thought bringing him down would result in a 15-yard penalty. There’s a good chance he’s right. The referee Sunday was Larry Nemmers, whose officiating crew calls more penalties than any other group of officials in the league. Giants fans remember Nemmers as the referee who called the Giants for 19 penalties last year in their loss to the Seattle Seahawks.

Kiwanuka got his arms around the 6-foot-5, 228-pound Young — who is one of the biggest and strongest quarterbacks in the NFL — and to tackle him, Kiwanuka would have had to use all of his body weight to drive him into the ground. But if he had done that, Nemmers could have flagged him for unnecessary roughness. Earlier this year the Cincinnati Bengals were in an almost identical situation to the Giants: They led the Tampa Bay Buccaneers late in the fourth quarter, and when Cincinnati defensive end Justin Smith sacked Tampa Bay quarterback Bruce Gradkowski, it looked like Cincinnati would win. But the referee gave Smith a 15-yard penalty, explaining that he had violated the rule against using the weight of his body to drive the quarterback into the ground. That penalty gave the Buccaneers a second chance, and they drove for the winning touchdown. After that game, an exasperated Cincinnati coach Marvin Lewis said, “I guess you’ve got to cuddle him to the ground.”

Kiwanuka had his head down as he drove his shoulder into Young’s body, and he couldn’t tell whether Young had passed. If it had turned out that Young did throw the ball, but Kiwanuka tackled him anyway, the post-game stories would have said Kiwanuka cost his team the game with his stupid roughing the passer penalty. Kiwanuka’s play was simply the product of the NFL’s longstanding policy of writing rules designed to protect quarterbacks, and the latest piece of evidence that the league should revisit those rules. Although outlawing blows to the head is wise (Giants cornerback Frank Walker was correctly flagged for a helmet-to-helmet hit on Young along the sideline earlier in the fourth quarter), the NFL should let defenders play tackle football the way they’ve played it their entire lives.

Kiwanuka’s non-sack seems destined to be remembered as the signature moment of the Giants’ epic collapse. But his mistake is far more understandable than the underthrown pass by quarterback Eli Manning a few minutes later. With the score tied, Manning wanted to drive the Giants into position to kick a game-winning field goal, but his primary goal should have been avoiding a turnover. Instead, he passed to David Tyree along the right sideline, throwing low enough that Tennessee cornerback Pacman Jones was able to jump in front of Tyree and pluck the ball out of the air. It was a terrible mistake, costlier than Kiwanuka’s, and a play that Manning, who was making the 34th start of his career, should have known not to make.

Manning’s mistake wasn’t for a lack of effort, which can’t be said for the play by receiver Plaxico Burress on Jones’s prior fourth quarter interception. Burress ran a lackadaisical route on that play and Manning overthrew him, and instead of using the height and jumping ability that led the Giants to sign Burress to a lucrative contract last year, he passively watched the ball go past him. Even after Jones made the interception, Burress continued with his halfhearted effort, failing to tackle Jones and allowing him to pick up 26 yards on the return. At the time of Burress’s lazy route, the Giants had a three-touchdown lead, but that interception set up the first Titans touchdown and paved the way for the meltdown. Ultimately, Kiwanuka’s fourth quarter blunder was understandable, Manning’s was inexplicable and Burress’s was unforgivable.

After the game Giants coach Tom Coughlin said, “We’re going to be sick about this one forever.” But if he allows his team to dwell on this loss, Coughlin will have made the greatest mistake of them all. The Giants’ current threegame losing skid has coincided with the Dallas Cowboys’ three-game winning streak, and the Giants now find themselves a game behind the Cowboys in the NFC East. The Giants have already beaten the Cowboys once this year, and with Dallas coming to the Meadowlands on Sunday, the Giants can move back into first place. It’s Coughlin’s job to have his team ready to do that.

Kiwanuka sounded just as sick about the loss as Coughlin after the game, saying he had cost the Giants the game. But Kiwanuka is wrong. Both the NFL’s silly rules and some of his teammates deserve more blame than he does. And if the Giants don’t regain their focus this week, Coughlin will deserve more blame than anyone.

Mr. Smith is a writer for FootballOutsiders.com.