Pressure is one of the most difficult things in the NFL for a quarterback to deal with. The reason that it's so difficult for a quarterback to handle is that it's (usually) not his fault that he's handling it. Due to this, it stands to reason that a "perceived cumulative pressure" (that is, phantom pressure coming from numerous hits) would affect a quarterback's performance as well. Nathan Jahnke of Pro Football Focus recently wrote an article exploring this phenomenon, and he found that while the average quarterback suffers about a 0.5% hit in accuracy per sack taken, Brees is among the quarterbacks with a greater than 1% hit per sack.
The other three quarterbacks were Matt Ryan, Jake Locker, and Mark Sanchez. This data was collected over the past 6 years, so the only real constant is the quarterbacks themselves. To explain Brees's decrease in accuracy after sacks, one must look at his general tendencies. First of all, he's a pure pocket passer. Brees will only flush out if he truly has to, thus making forming a pocket far more important. If he starts to feel the pocket collapsing, even if it is a phantom collapse, he's more prone to make mistakes.
Furthermore, Brees has to go through his progressions. He is not, nor will he ever be, a one read quarterback. It's imperative that he not only has time to make his reads, especially against zone coverages that are frequently implemented against him, but that he also has throwing and vision lanes created by his line.
Which leads into Brees's last attribute that may contribute to this: his height. At 5'11", Brees can't just see over his offensive line like players like Peyton Manning or Tom Brady. He needs to keep his feet moving and he needs to know his players' routes better than the average quarterback. If phantom pressure gets factored into that equation, then it's too much for him to think about, and he'll either take more sacks or start forcing throws into impossible windows.
Here are the stats on Brees's passing accuracy after a certain number of sacks:
The steep drop-offs from 2-3 and from 4-5 are what skew the numbers, with the Mean percentage in drop-off being 1.5% Many people talk about how quarterbacks have "short term memory," but like any position group, they'll remember when they're hit over and over again. In 2013, Brees was sacked 37 times, 10 more than he had been single-season at any point in his career. One of the worst games came against the Tampa Bay Buccaneers. He was sacked 4 times, and he went only 26-46 passing in that game with 2 interceptions, in addition to posting a miserable 67.5 Passer Rating, his lowest of the season.
This play occurred the series after Brees took his second sack of this game. The Saints are in a spread formation with 3 receivers, whereas the Bucs are in a 3-3-5 defensive set.
The circle is indicative of how much room Brees has on this play. The Saints run almost exclusively 5-7 step dropbacks, in an attempt to let Brees better survey the field. On this play, he has at least 5-7 yards in every direction, giving him a clear view on everything unfolding around him.
However, rather than letting the play unfold, Brees steps into pressure and forces a throw into an impossible area for Lance Moore. He very nearly gets picked off on this play. Brees had time to actually go through a few more of his progressions on this play, but rather than letting the play develop as the playbook would dictate, he simply lets the ball fly and nearly pays the ultimate price for it.
The Saints later come out in the third quarter in a basic shotgun formation, again singleback. The Buccaneers are running a 3-3-5 (as they did throughout the game) with two safeties high and Revis isolated on the Y receiver.
Once again, the protection is solid on this play, with Brees dealing with a 5-7 yard pocket. However, Brees only makes one read on the play, looking at Graham the entire time. The red line is indicative of where the receiver breaks down and shakes his corner, in this case Lance Moore out of the near side slot. Brees never looks at him, thus making Graham the only option on the play.
The result is, of course, an incompletion from Brees to Graham. The DB on Graham knew where Brees was going the entire time, and therefore was able to read Graham's breakdown and break up the pass. Furthermore, the defensive tackle gets his hand on the ball, and it's easy for him to time his jump and get himself into Brees's throwing lane by simply bull rushing up the middle. Brees steps into the pocket, thus making it so that all that the pass rusher has to do is throw his hands up. The result is a duck to Graham that the defensive back easily breaks up. Brees doesn't work as a one-read quarterback, the concept flies in the face of the Saints entire offense. Watch Brees's eyes throughout the duration of the GIF above, he stares down Graham throughout the entire play, rather than surveying the field and finding the open receiver as he normally does so well.
This is the play that very nearly cost the Saints the game. It's a base singleback vs. a 4-3 under that is feigning pressure up the middle.
At the snap, the would-be blitzers are going to bail into a basic Cover 2 defense. The linebackers are covering the intermediate routes while the secondary takes away deep passes. The flats are left unattended on the play. At the snap, it looks something like this:
The Buccaneers DBs have not yet met the receivers in their zones. However, the Saints do not have an outlet hot read on this play. At this point, this is in essence a busted play. Brees is looking at Graham, but there is no seam to be had.
Brees actually does face some pressure up the middle, you can see Adrian Clayborne right in his face at the time of release. However, his running back (Pierre Thomas) wisely broke off his route in a wide open stretch of field in an attempt to give Brees a hot read, marked in red. Brees misses him, and tries to force the pass to Graham, with the trajectory of the ball marked in black. Watch where the ball is compared to Foster:
Too easy for a player of Foster's talent, and way too easy for literally anyone of NFL caliber. Foster doesn't even have to move, he just has to follow Brees's eyes. Brees is actually running back up the field before the ball is even caught. Foster had Brees's number all game, playing a great defensive game, but Brees telegraphed every ball he threw his way, because the Bucs were able to get pressure on him early.
The Saints would obviously end up winning this game, but that was mainly due to the defense holding Tampa to 14 points. Give Brees credit for orchestrating that last drive, but this game is the best indicator of what happens if you get into his head. When it feels like the pocket is collapsing around you every single play, a quarterback will invariably make mistakes. Brees is a truly great quarterback, but he needs help up front. If he's constantly taking blind side hits, then that's one more thing that he has to think about. In an offense as complex as the one that New Orleans runs, that can break down a team.
Brees is one of the best in the game, but pressure affects him as much as anyone else, if not moreso. He needs help from his offensive line in order to succeed. There's a reason the Saints won the Super Bowl with him in 2009: Brees wasn't doing everything and he was rarely hit. He took only 20 sacks, and he's never had less than 24 since. If the Saints are going to go all the way again with him at the helm, he'll need better protection than he's had recently, and he'll need far better protection than what he had last year. He can lead the team, but he can't do everything himself. People tend to forget this about quarterbacks.
**Special Thanks to Clay Wendler for the GIF help