This past off-season, the Chargers had a dilemma. Would they re-sign Drew Brees, who had developed into one of the league’s top passers and become the team’s locker room leader? Or would they allow him to leave, handing their championship-quality team over to Philip Rivers, an unproven firstround prospect who had been taking up space on both the bench and salary cap for two years.

General manager A.J. Smith’s decision was made somewhat easier when Brees suffered a shoulder injury in the final game of the season. But still, many observers criticized the move. The Chargers had perhaps the league’s best running back and tight end in LaDainian Tomlinson and Antonio Gates, and they had a great defense. Sure, Rivers was a top-five draft pick, but so were Joey Harrington and Tim Couch once upon a time, and they never developed into stars. Why dump a proven leader for an unproven commodity with no career starts in the NFL?

The season is only half over, but already Smith can feel vindicated. The Chargers are 5–2,tied with Denver atop the AFC West. The Chargers lead the AFC with 216 points, and both losses came by just three points apiece. As for Rivers, he ranks fourth in the NFL with a 97.6 passer rating and is one of only three full-time quarterbacks to complete more than 65% of his passes. He’s also been consistent; Rivers only has one game with a passer rating below 80, and that came against Baltimore, the top defense in the league this year.

Just as important as Rivers’s statistics is the anecdotal evidence that shows him playing at a veteran level despite starting just seven NFL games. He has pocket presence, rarely panicking in the face of a strong pass rush. His passes have touch, often delivered in the precise location where only his receiver, and not a defensive back, can get to it. Teammates rave about his leadership in the huddle and publicly pressured head coach Marty Schottenheimer to open up his offense and let Rivers throw more. Television analysts from John Madden to Merril Hoge have compared Rivers to Hall of Famer Dan Marino.

It’s impossible to predict the development of quarterbacks with perfect accuracy, but in retrospect, Rivers’s maturation into one of the league’s top passers should have surprised no one. Scouting reports on Rivers emphasized his accuracy, instincts, and leadership skills. He was known for his ability to go through progressions and find secondary receivers, often a problem for college quarterbacks who graduate to the pro level.

The scouting reports are backed up by research by Macalester College student David Lewin, presented in “Pro Football Prospectus 2006.” Quarterbacks taken in the top two rounds of the draft can be projected with remarkable accuracy using just two statistics: collegiate games started and completion percentage. The lowest-rated quarterbacks by this measure include Ryan Leaf, Jim Druckenmiller, and Akili Smith, all washouts. The highest-rated quarterbacks, in order, are Ben Roethlisberger, Philip Rivers, Peyton Manning, and Carson Palmer. Rivers started 51 games at North Carolina State, and his senior-season completion rate of 72% is an ACC record.

And while most rookies struggle in their first exposure to the NFL, Rivers isn’t a rookie. He has spent two seasons on the San Diego bench, watching film in meetings, working out with NFL coaches, and throwing in practice to NFL receivers. The last top quarterback prospect to sit out two years before finally playing was Chad Pennington. Pennington’s constant injuries tend to obscure that he was one of the top quarterbacks of 2002 from the day he entered the lineup for the first time.

Entering the season, the conventional wisdom said the Chargers were a Super Bowl contender, but their novice quarterback would likely hold them back. Instead, Rivers is one of the main reasons why the Chargers are a Super Bowl contender — but the unit most people considered the heart of the Chargers could end up holding them back.

Last year, San Diego had one of the best front sevens in football. Opposing running backs averaged just 3.6 yards a carry, and 46 sacks put the San Diego pass rush near the top of the league. Linebacker Shawne Merriman was Defensive Rookie of the Year, and nose tackle Jamal Williams went to the Pro Bowl.

This year, Williams seems to be the only stable part of the unit. The pass rush is still ferocious and leads the league with 26 sacks, but opposing running backs are gaining a league-average 4.0 yards a carry. A red-zone defense that was average in 2005 has been the worst in the league so far this season.

Part of the problem has been personnel. Linebacker Steve Foley was shot by an off-duty cop in a still-mysterious encounter before the season began. His replacement, Shaun Phillips, has missed the last two games with a calf injury. Defensive end Igor Olshansky will miss at least three weeks after knee surgery.

The biggest blow is the four-game suspension to Merriman after he tested positive for a banned substance. Though Merriman claims the positive drug test was caused by a tainted legal supplement, most reports have tied the test to steroids. Merriman played on Sunday against St. Louis after appealing his suspension, but he dropped that appeal yesterday to prevent the possibility that a delayed rejection of the appeal would keep him from playing both games against Denver, November 19 and December 10. Merriman can have no contact with anyone from the team for the next four weeks, and the Chargers will have to go on the road to face Cincinnati and Denver without him.

With the exception of Foley, all these players will return in time for the stretch run. But the Chargers simply can’t afford any more close losses if they want to grab home-field advantage in the postseason. Climate is not destiny, but warm-weather teams have a terrible historical record in the playoffs, and it is hard to see the Chargers winning in Denver, Baltimore, or Foxboro on a cold day in January.

However, what if home-field advantage belongs to an indoor or warmweather team, and all of San Diego’s defensive pieces are back in place? In that case, it would be very easy to picture Philip Rivers leading the San Diego Chargers into the Super Bowl. And for A.J. Smith, that would be the best vindication of all.

Mr. Schatz is the editor in chief of FootballOutsiders.com.