A promising young player replaced an established veteran starter for the Jets on Sunday, and how that young player performs for the rest of the season will go a long way toward determining whether coach Eric Mangini has his team on the right track.

We’re not talking here about Kellen Clemens replacing Chad Pennington as the Jets’ starting quarterback. We’re talking about David Harris replacing Jonathan Vilma at inside linebacker.

Although Pennington got the largest share of the blame and ultimately lost his job because of the Jets’ 1–7 start, the defense was a bigger culprit. And it stood to reason that the defense could get even worse when Vilma, the Jets’ 2004 first-round draft pick, was placed on injured reserve last week. Vilma hadn’t missed a start since the third game of his rookie year, and replacing a player of his considerable talent isn’t easy.

But Harris, this year’s second-round pick, was brilliant in Vilma’s place in Sunday’s loss to the Buffalo Bills. He flew all over the field, blitzing on one play, dropping into deep pass coverage on the next, stepping up to stuff a running back after that. Harris was the game’s leading tackler, with an incredible six tackles either at or behind the line of scrimmage. He also fought through a double-team to record the Jets’ only sack of the day.

There’s no hope for the Jets to have a successful season in 2007, not when they’ve already lost more games this year than they did in all of 2006. But just as a promising performance over the second half of the season by Clemens can signal good things for the Jets’ future on offense, so can Harris show over the final eight games that the defense Mangini is building has potential, even if it’s not close yet to being where it should be.

Heading into this year’s NFL draft, the Jets had inside knowledge of what kind of player Harris is because their linebackers’ coach, Jim Herrmann, coached him at Michigan. Harris knew what was expected of him from Herrmann, and Herrmann knew that Harris had the talent necessary to fit with the 3–4 defense that Mangini is trying to implement.

That makes Harris quite a bit different from Vilma, an extremely gifted player whose talents have never looked well-suited to Mangini’s defense. Vilma is an incredibly fast, hard-hitting linebacker, but he struggles at shedding blockers, which 3–4 inside linebackers need to do to be successful. Harris isn’t as explosive an athlete as Vilma, but he excels where Vilma struggles — fighting off blockers — and he looks better at stopping the run right now than Vilma has ever been.

Good coaches are supposed to be able to find ways to use talented players, and the fact that Vilma went from a budding star in his two years playing in Herm Edwards’s defense to struggling in his two years playing in Mangini’s defense is a major failing of Mangini’s. If Harris continues to play well for the rest of this season, the best solution both for the Jets and for Vilma could be trading him in the off-season to a team that runs a defense more consistent with his skills. Vilma’s value in a trade, however, is diminished by his injury; teams won’t give the Jets a stockpile of draft choices for a player who’s coming off season-ending knee surgery.

No matter how well Harris plays for the rest of the season, the Jets still have a long way to go before they’ve acquired the kind of defensive personnel that Mangini needs to turn the defense into anything like the one he coached as an assistant with the New England Patriots. Most of the players on the Jets’ front seven look better suited for a 4–3 defense than Mangini’s 3–4. As a player who looks like an ideal fit for what Mangini wants to do on defense, Harris is the exception.

Against Buffalo, Harris had one key advantage: The Bills didn’t know what to expect from him. On the strength of his performance Sunday, teams will start game planning more specifically for Harris. He’ll have to show that he can still tackle running backs behind the line of scrimmage when he’s the focal point of the opposing team’s run blocking, and that he can still get to the quarterback when the opposing offensive line has spent all week preparing to block him.

Given the increased attention he’ll get from opposing offenses, Harris can’t be expected to make the kind of impact he made against Buffalo every week. But if he can even come close to playing that way over the second half of the season, he’ll be one of the few reasons for optimism coming out of an otherwise dismal Jets season.

Mr. Smith is a writer for FootballOutsiders.com.