When the Denver Broncos begin their title defense with Trevor Siemian under center on Sept. 8, they’ll be in a rare spot for reigning Super Bowl winners. Last year’s starting QB, Peyton Manning, retired in March, guaranteeing Denver would become only the fifth champ of the Super Bowl era1Since opening day of 1967, because there was no defending Super Bowl champ in Week 1 of the 1966 season. whose opening-day primary quarterback2For the purposes of this entire article, I’ll be referring to a team’s “primary QB” — the player who led the team in dropbacks in a game — as a proxy for its starter, since Pro-Football-Reference’s Game Finder doesn’t provide data on which QB started each game. during the next season was not the same signal-caller who led the way on Super Sunday.And even among that group of new QBs, Siemian is almost uniquely inexperienced. Assuming he ends up being Denver’s primary passer in Week 1, he’ll join another Bronco — Brian Griese, who replaced John Elway in 1999 — as the only opening-day primary QBs for a defending champion whose prior career featured both zero career starts and zero points of career Approximate Value (AV). By contrast, the average defending champ through 2015 returned a QB with 74.8 career starts and a lifetime AV of 66.5, meaning they were both pretty seasoned and pretty good. And that experience has traditionally been an advantage in the pursuit of a repeat Super Bowl bid. All else being equal, teams whose opening-day QBs have more career AV under their belt tend to perform better in the passing game. For instance, a simple regression would predict a 70-AV QB’s team to pass the ball with a half-standard deviation greater efficiency3According to Football Outsiders’ defense-adjusted value over average (DVOA) metric. — the equivalent of four-fifths of an extra win over a full season — than that of a zero-AV QB, while a more complex fit suggests the same edge in experience could be worth even more, as much as a full standard deviation of passing efficiency or 1.7 wins. So, given Siemian’s lack of experience (let alone his meager draft pedigree), we’d expect him to lead a passing attack that’s fairly lousy. But even if Siemian spearheads a passing offense with a DVOA index4Which sets DVOA on a common scale where 100 is league average and a standard deviation in either direction is 15 points. of 90, two-thirds of a standard deviation worse than NFL average — as the average zero-AV opening day starter has during the Super Bowl era — it would represent an improvement to the Broncos’ passing attack.Simply put, Denver won the Super Bowl last season in spite of its passing offense, not because of it. In terms of passing efficiency, the Broncos were an entire standard deviation worse than league average last season, which equated to the NFL’s eighth-most inefficient aerial attack. And Manning himself was awful, producing the worst season of any Super Bowl QB in history. His backup, Brock Osweiler, wasn’t great either, but if Siemian’s 2016 performance can mimic Osweiler’s more than Manning’s, the Broncos will still be better through the air than they were a year ago.And with a defense as dominant as the Broncos still possess, that might be enough to contend for another Super Bowl, even given Siemian’s historic inexperience under center.