Something unheard of happened this weekend. Something we haven’t seen all year; in fact, something we haven’t seen since last December.Cody Kessler threw an interception.It was early in the first quarter. It turned out to be inconsequential in USC’s 42-14 win over Arizona State. But it’s still a noteworthy blemish on an otherwise flawless record for Kessler so far this season.Kessler can’t win the Heisman anymore.Joking. As fellow Daily Trojan columnist Regan Estes discussed yesterday, Kessler absolutely belongs in the discussion with the best players in the country. And he might even break the trend of recent USC quarterbacks and have a somewhat sustained NFL career. But I would say that regardless of USC’s performance the rest of the season, Kessler probably doesn’t have the athleticism to actually win the Heisman.Kessler might be the most underrated quarterback in all of college football. But he also might be the most overrated as underrated too. Everyone from ESPN to Bleacher Report to this publication have used that title for him, but he finally started getting some of the preseason hype he deserved this year as a preseason All-American and a mentioned player on a lot of Heisman watch lists.Last year, Kessler threw for 3,826 yards and 39 touchdowns while only racking up five interceptions. His completion percentage was 69.7 percent, and his quarterback rating was 167.1.His QBR tied for third in all of Division I-A, trailing only Oregon’s Marcus Mariota at 181.7 and J.T. Barrett of Ohio State with 169.8. His completion percentage was best in the conference and third best in the nation — Grant Hedrick of Boise State led the country with 70.8.His yardage was 11th best in the country and fourth best in the conference behind Mariota, Cal’s Jared Goff and Washington State’s Connor Halliday. His touchdown total was fourth best in the nation and second to Mariota’s 42 in the conference.In short, Kessler statistically was maybe the second-best quarterback in the country. The only players with comparable passing numbers were from outside of Power Five conferences — like Western Kentucky’s Brandon Doughty.Kessler was also the essence of reliablility last year. He never had bad games. He threw for fewer than 200 yards in just two games, and USC was 2-0 in those contests against Stanford and Arizona. He also never threw more than one interception in a game. Along with his remarkable reliability, he showed big play potential, with plenty of long balls over the top of the secondary for touchdowns.Kessler’s first four games this year look very similar to his first four games last year. They include very solid numbers in three big blowouts, as well as a very solid performance in a losing effort. Last year, he had 317 yards and four TDs with a 75 percent completion percentage in a defensive meltdown against Boston College. In this year’s defensive meltdown against Stanford, he was three TDs and 272 yards with a 78.1 percent completion rate.Last year, most of Kessler’s big numbers came from the worst defenses in the country, and there were notable drop-offs against tougher competition. He combined for about 18 touchdowns against Colorado, Washington State and Notre Dame but could only put together two TDs against Stanford, Arizona State, Arizona and UCLA. It’s too early to say if he will buck that trend this year. The only strong defense he’s really faced was maybe Stanford’s, and that loss was certainly not on him.With Mariota now in the NFL, Kessler is probably the best quarterback in the conference. The sample size is still small, but Kessler is statistically on top of Pac-12. His 15 touchdowns, 1,297 yards and 201.2 QBR all lead the conference. His 73.0 percent completion percentage is just fractions below Washington State’s Luke Falk — Kessler’s is technically 72.95 percent to Falk’s 73.03, but we’ll round up. Cal’s Goff is a close second and UCLA’s Josh Rosen has shown flashes of elite talent, but Kessler is the only one to show his tremendous reliability over a long period of time.The biggest knock on Kessler for me is that he is very much one-dimensional. Pocket passers aren’t necessarily worse than dual threat options, but having someone like a Mariota or a Brett Hundley who can pick up a couple first downs with his feet makes a huge difference over the course of a drive, game and season. Having a mobile quarterback totally changes the dynamic of an offense and puts so much more pressure on a defense. It’s why so many teams put their best athlete behind center, but almost no one would give that distinction to Kessler before guys like Su’a Cravens and Adoree’ Jackson.It’s the only reason why I doubt Kessler’s Heisman chances. Since Matt Leinart won the Heisman in 2004, the last time a USC quarterback has done so, eight quarterbacks and two running backs have won the Heisman — yes, including Reggie Bush in 2005. Of those eight, only one would really classify as a pocket passer — Sam Bradford in 2008. Troy Smith, Tim Tebow, Cam Newton, Robert Griffin III, Johnny Manziel, Jameis Winston and Mariota all have some running ability that not only attracts voter attention but also makes a difference on the field.If the Heisman goes to a QB again this year, it will probably be someone like TCU’s Treyvon Boykin or Clemson’s Deshaun Watson who are real dual threats. This year, though, it will probably go to a running back because LSU’s Leonard Fornette has looked like the best player in the country.That doesn’t mean that Kessler is not an exceptional quarterback capable of taking his team to the playoff. In fact, if he can lead USC to a conference championship, he’ll almost certainly be a finalist for the sport’s top honor.As long as he doesn’t throw another interception this year.