The New England Patriots opened as the favorite in their Super Bowl matchup against the Philadelphia Eagles, but that doesn’t mean it will be easy for Coach Bill Belichick and quarterback Tom Brady to earn their sixth championship ring together. In fact, it could become very difficult if Eagles quarterback Nick Foles can exploit a glaring weakness in this year’s Patriots: a poor pass defense that is one of the worst of any Super Bowl team coached by Belichick in New England.

The Patriots allowed opposing quarterbacks to produce an 89.5 passer rating. Since 2001, Belichick and Brady’s first title together, only the 2008 squad, which failed to qualify for the postseason, was worse (89.8). The NFL is a passing league, so not being able to effectively hamper opposing quarterbacks is a problem. It’s also why net passer rating differential, dubbed the “Mother of All Stats,” has been a reliable litmus test for Super Bowl-caliber teams: It isn’t enough to overwhelm an opponent on offense; you also have to limit its ability to throw down the field on defense.

Since 2002, 22 of the 30 Super Bowl participants had a passer rating differential among the top five in the NFL, including 10 of the past 15 winners. The outliers among the winners include the 2005 Pittsburgh Steelers (seventh), 2007 New York Giants (24th), 2011 Giants (12th), 2012 Baltimore Ravens (12th) and 2015 Denver Broncos (18th).

Looking at it another way, New England’s pass defense allows opponents to score 3.3 more points per game than expected after taking into account the down, distance and field position of each throw. Only the 2011 Patriots, who were upset by the Giants in Super Bowl XLVI, were worse in the Belichick/Brady era.

Perhaps this wouldn’t be as concerning if Foles hadn’t just dismantled the Minnesota Vikings in the NFC championship game. The Vikings entered that game allowing a passer rating of 74.0, but Foles turned in a career performance, completing 26 passes for 352 yards and three touchdowns (good for a 141.4 passer rating). Per Elias, Foles was the first quarterback in NFL history with completions of more than 40 yards to three different receivers in a postseason game.

Most impressive was how Foles handled the Vikings’ pass rush, which was widely considered among the best in the NFL. Foles completed 7 of 10 attempts under pressure for 139 yards and two touchdowns, producing a sparkling 152.1 passer rating (out of a possible 158.3). That’s a remarkable turnaround, considering Foles had a 23.8 passer rating under pressure during the regular season. Yes, that is as bad as it sounds: An incomplete pass produces a passer rating of 39.6.

Nick Foles … 2017 regular season vs. Vikings in NFC championship game
Facing no pressure 107.8 119.7
Under pressure 23.8 152.1

Foles also extended the defense with deep throws, completing 4 of 6 passes that went at least 20 yards in the air for 172 yards and two touchdowns. He was 2 for 15 on those throws before Sunday, compiling 52 yards and no touchdowns while also throwing an interception.

So it all comes down to how good the Patriots defense is on Super Bowl Sunday. In wins this season, including the playoffs, the Patriots allowed a passer rating of 83.2. That jumped to 130.6 in losses, a number that Foles topped against the Vikings.

One more big game from Foles, and we could be looking at a historic upset.

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