Houston Texans quarterback David Carr doesn’t have much in common with Indianapolis Colts quarterback Peyton Manning. They’re both starting quarterbacks and former first overall draft picks, but Manning is heading for the playoffs in his familiar perch atop the league’s passer rating list, while Carr is struggling through another last-place season.

When they played the Jacksonville Jaguars this season, however, Carr and Manning reversed positions. Each played Jacksonville twice, and in those games Carr completed 41 of 66 passes for 391 yards with two touchdowns and no interceptions, while Manning went 39-of-81 for 532 yards, with one touchdown and one interception. Against Jacksonville this season, Manning’s passer rating is 68.6 and Carr’s is 88.6. In their 12 other starts, Manning’s passer rating is 105.0 and Carr’s is 80.8. Those numbers say less about Manning and Carr than they say about the Jaguars, who have a defense that could go on a dominating run in the next six weeks and lead the team to the Super Bowl — or collapse in the next two weeks and fail to make the playoffs.

Take the season as a whole, and Jacksonville’s defense is among the best in the league, every bit as good as those of the Chicago Bears and Baltimore Ravens. Jacksonville ranks second in the league in yards allowed and touchdowns allowed, and the Jaguars’ stifling pass defense has held opposing quarterbacks to a completion rate of 54.5%, lowest in the league, while allowing just 10 touchdowns and intercepting 18 passes. Jacksonville’s defense is strong, but it would be stronger if it would stop mixing in the occasional lousy performance against the likes of Carr or the Washington Redskins’ Mark Brunell.

Jacksonville coach Jack Del Rio, who spent 11 years as an NFL linebacker, has built that strong defense despite having three little-known starting linebackers: Daryl Smith, who has manned the middle since starter Mike Peterson was lost for the season in Week 5; Clint Ingram, a rookie from Oklahoma who became a starter in October, and Nick Greisen, who spent his first four seasons with the Giants before signing with Jacksonville this year.

That unheralded trio has played well because Jacksonville has a solid defensive line. On running plays, the Jaguars’ front four, led by tackle John Henderson, occupy opposing offensive linemen, allowing the linebackers to roam free. On passing plays, Jacksonville pressures quarterbacks using just the four linemen, which allows Smith, Ingram, and Greisen to drop back into coverage instead of blitzing. Defensive end Bobby McCray wasn’t even a starter until Week 5, but he has developed into one of the league’s best pass rushers, recording nine sacks this year. With the pressure helping him, Rashean Mathis has become one of the best cornerbacks in the league. Mathis is tied for third in the NFL with seven interceptions and was the only Jaguar selected to the Pro Bowl.

But that maddening inconsistency makes Jacksonville a long shot to get far in the playoffs. For the 8–6 Jaguars, reaching the Super Bowl would likely require beating both the New England Patriots on Sunday and the Kansas City Chiefs next week, then winning three straight road playoff games. That is certainly possible — the Pittsburgh Steelers went on a similar run last year on their way to the title.

Although the Jaguars suffered several injuries on defense, including tackle Marcus Stroud missing five games with an ankle injury and losing three top players — Peterson, defensive end Reggie Hayward, and strong safety Donovin Darius — for the season, injuries don’t explain Jacksonville’s inconsistency. The personnel was basically the same in three consecutive October games when the defense looked terrible in a 36–30 loss to the Redskins, then looked great in a 41–0 win over the Jets, then looked terrible again in a 27–7 loss to the Texans.

If the Jaguars play against New England quarterback Tom Brady on Sunday like they did against Manning, they’ll have the look of a team that could head to Miami for the Super Bowl on February 4. If they play against Brady like they did against Carr, their season is all but over.