For most of the season, the Denver Broncos’ bid to repeat as Super Bowl champs looked like it would be relatively smooth sailing. The Broncos opened the season as the best team in the NFL according to FiveThirtyEight’s Elo ratings, and before this weekend, our playoff predictions (which are based on Elo) never gave the team less than a 74 percent chance of making it to the postseason. Going into Sunday night’s game against the division-rival Kansas City Chiefs, Denver had a healthy 76 percent chance of punching a return ticket to the playoffs.

But after losing to K.C. when Chiefs kicker Cairo Santos banked in a game winner off the left upright as time expired, Denver unexpectedly finds itself in a precarious situation. The Broncos are in third place in the AFC West, a game behind the Chiefs and two behind the Oakland Raiders, and our model gives Denver only a 54 percent chance of returning to the playoffs. All of a sudden, the Broncos’ title defense is in doubt.

How did Denver get into this predicament? For one thing, the team’s quality of play has slipped ever so slightly compared with last season. At this stage of the 2015 season, Denver ranked fourth in the NFL with a 1661 Elo rating, and our model assigned them a 10 percent probability of winning the Super Bowl. This year, despite competing with fewer great teams, the Broncos rank fifth with a 1606 Elo and a 4 percent Super Bowl probability. If they instead sported that 1661 rating from last season, this year’s Broncos would be second in the NFL, nipping at the New England Patriots’ heels for the right to be No. 1.

For a while this season, the Broncos’ defense looked like it might be able to keep steady with the all-time great D the team assembled last season, and Denver still has the best defense in football according to expected points added. But like most of history’s dominant defenses, the Broncos’ D has experienced some reversion to the mean, and the team is now getting 4.8 EPA per game out of its defense rather than the 6.6 they got last season. As expected, Denver’s offense has improved under first-year starting QB Trevor Siemian (it would have been hard not to, given how poorly Peyton Manning played last season), but it hasn’t been enough to offset that defensive regression and a drop-off on special-teams after adjusting for strength of schedule. As a result, the Broncos are no better off now than they were last year.

EXPECTED PTS ADDED/GAME RANK CATEGORY 2015 2016 2015 2016 Passing offense -1.1 +2.2 27 20 Rushing offense -1.6 -1.8 20 23 Offense -2.6 +0.7 25 18 Passing defense +4.8 +5.7 1 1 Rushing defense +2.5 -0.5 5 27 Defense +6.6 +4.8 1 1 Special teams -0.3 -1.4 25 26 How the 2015 and 2016 Broncos compare 2015 numbers are for the full season; 2016 numbers are as of Nov. 27.

Offensive and defensive EPA won’t necessarily match the sum of rushing and passing EPA on each side of the ball because penalties are tracked in a separate category. Source: ESPN Stats & Information Group

And the rest of the AFC West has improved — by leaps and bounds, in some cases. After Week 12 of the 2015 season, the Broncos’ division rivals — the Chiefs, Raiders and San Diego Chargers — had an average Elo rating of 1489, well below the NFL-wide average of 1505. K.C.’s Elo rating ranked seventh in the league, and the Chiefs were five victories into what would eventually be an 11-game winning streak. But the Raiders and Chargers were each in the league’s bottom third, and the AFC West was fourth among the NFL’s eight divisions in overall average Elo.

This year’s AFC West, by contrast, easily ranks first in all of football in average Elo. The Chiefs’ Elo rating has improved by 32 points from where it was this time last season, the Chargers’ by 68 points and the Raiders’ by a stunning 139 points — that makes Oakland the league’s second-most-improved team over that span (trailing only the Dallas Cowboys). With an average Elo of 1578, the 2016 AFC West is on pace to be the NFL’s 10th-best division since the league merged with the AFL before the 1970 season.

YEAR DIVISION TEAMS AVG. ELO 1 2013 NFC West 4 1624 2 2007 AFC South 4 1608 3 1984 AFC West 5 1600 4 2004 AFC East 4 1590 5 2014 NFC West 4 1588 6 1976 AFC Central 4 1583 7 1975 AFC Central 4 1581 8 1970 NFC Central 4 1581 9 2008 NFC East 4 1581 10 2016 AFC West 4 1578 The NFL’s strongest divisions, 1970-2016 Based on average Elo rating of the division’s teams. Source: Pro-Football-Reference.com

All of this has conspired to make the Broncos’ bid for a championship repeat far more daunting than it seemed before the season began. Only 12 teams with an Elo rating as high as Denver’s current mark have missed the playoffs since the merger, but right now, it’s basically a tossup as to whether the Broncos will become No. 13. And because the Broncos’ playoff odds would have been much higher if they’d won or tied Sunday night, their playoff fate might end up depending on — of all things — the few inches that determined where Santos’s kick went after glancing off the goalpost in overtime.

Check out our latest NFL predictions.