Brock Purdy and the Allure of the Unknown Backup QB

After an impressive debut, what does the film say about 49ers rookie QB Brock Purdy and his chances of continued success?

Brock Purdy entered the Dolphins game with the San Francisco 49ers down 7-3, ending in a 33-17 San Francisco win. Not unlike Mike White a few weeks ago, Cooper Rush at the beginning of the season, or Tyler Huntley a season ago, a backup QB with little experience brings hope when the team produces with them on the field. The problem is that very few of them turn out to be Taylor Heinicke, who is only a mediocre starter himself.

Usually, these flash-in-the-pan backups crash and burn spectacularly. Sometimes they can go on a mini-run of four or five games before it happens, but it’s nearly inevitable. Even White, whose tape against Chicago was exceptionally clean, struggled the following week against Minnesota.

But could Purdy buck that trend and finally be the backup behind Jimmy Garoppolo, which doesn’t end as an abject failure? Have the 49ers finally finished building a QB-proof offense?

Brock Purdy by the Numbers

Nobody in the NFL had a lower average target depth in Week 13 than Purdy. His 5.3 mark was 0.4 lower than Tyler Huntley, another backup coming into a game he didn’t start. Among the 30 quarterbacks who posted at least 20 attempts, Purdy ranked 26th in ANY/A, 13th in catchable rate, 16th in on-target rate, and 10th in positive EPA rate.

While EPA is not necessarily a QB-independent stat, it can show how well a QB operates within the scope of the offense. For instance, nobody considers Jimmy Garoppolo a top-5 QB, and most wouldn’t consider him inside of the top 10. However, he ranks sixth in adjusted EPA-per-play in 2022. And since 2020, only Josh Allen, Patrick Mahomes, and Aaron Rodgers have higher marks.

We need to judge Purdy based on that mark — overall offensive efficiency. However, we can use his film from the Dolphins game to project whether he can replicate positive offensive results.

Tale of the Tape

The Dolphins’ defense didn’t throw many wrenches at Purdy from a coverage perspective, but they played the part of Patches O’Houlihan rushing the passer. They didn’t throw a ton of exotic blitzes at Purdy, but they crowded the line of scrimmage and confused the San Francisco pass protection multiple times throughout the night.

Handling Pressure

The way Purdy navigated such situations was exactly what you’d like to see from a young QB.

Miami shows six at the line of scrimmage but only brings four. San Francisco elects to slide left, which would make Jaelan Phillips the free rusher in a six-man rush against five blockers.

Even though they only bring four, Josh Boyer drops two defenders with numbers in the 90s into coverage. Shockingly, the two Miami defenders don’t get any depth in their drop, leaving the middle of the field wide open.

But Phillips still comes free through the line. Purdy’s initial read brings him to the left side of the field, but he feels the pressure coming immediately and brings his eyes to the middle, delivering a strike while getting hit.

If one has never played the position before, it’s not easy to understand how quickly things must process through the brain and communicate to the body. QBs are practically seeing shapes and flashes of jersey colors, and it takes confidence bordering on arrogance, plus rapport with the pass catchers to consistently deliver passes.

Processing and Delivering

Right now, that “confident processing” is not consistently there for the young QB. That’s the biggest difference between Purdy and Garoppolo, and it’s what can turn tight-window throws over the middle into interceptions against zone coverage.

Here we are with the absolute basics of QB play. Reading and delivering slant-flat against Cover 3 should be so engrained that Purdy should be able to sleepwalk through this rep. It’s arguably the very first concept you learn in pee-wee football when a young QB barely has enough arm to get the ball outside of the tackle box.

The slot defender is the key here. If he clears the slant on his way out to the flat, throw the slant. If he sits more square, throw the flat route. The outside cornerback is somewhat of an afterthought here because he has a deep responsibility and shouldn’t be in a position to defend the slant.

The final piece of the puzzle is the playside linebacker. If he darts at a 45 into the curl zone, then the drag from the opposite side is open. But instead, the linebacker drops straight back into the hook and isn’t a factor in the coverage against the slant.

Here, nobody is. And once Purdy sees the slot sliding to the flat, this ball should be out to the slant.

A QB needs to have confidence in his receivers, and that’s hard to gain without receiving hundreds if not thousands of reps with them in practice and in voluntary workouts during the offseason.

Option routes are part of every offense, but against zone coverage, receivers work to open space within the zone instead of simply running straight and turning around. Purdy must have confidence in the when and where of each receiver, and he doesn’t yet have that.

In the above example, Purdy wants the backside slant, but the edge defender drops back into coverage, taking that window away. Purdy’s eyes take him backside to the spot concept and to Deebo Samuel finding space against the zone coverage.

As Deebo flips around, the ball should be coming out toward the left hash, away from the coverage on the inside. But all it takes is an extra hitch and this goes from easy completion to a dangerous tight-window throw.

Being a tick late is all it takes at the NFL level. It won’t always be the difference between completion, incompletion, and an interception, but it very well can be. And being late was a consistent theme for Purdy in his first NFL action, as it should be for a seventh-round rookie!


Could Purdy’s trial-by-fire continue to look as easy as it did against Miami? Anything is possible. But for every Tom Brady, there are 100 late-round QBs that never materialized.

Because the 49ers are unbelievably talented with the ball in their hands, there’s a chance the offense truly is QB-proof, to an extent. That is, if Purdy’s indecision leads to punts and not turnovers, their roster might be good enough to overcome the downtick in offensive production. He’ll need to hit on passes like in the above video as a professional QB.

Yards after the catch are also a product of timing and ball placement. And while Garoppolo can sometimes be prone to head-scratching decisions, his skill set allows the 49ers’ offense to work on schedule.

There’s a give and take with everything, including the above completion. The concept looks like Shanahan’s “Y Branch Nod F Zorro” from gun, although Brandon Aiyuk never gets to the corner.

The read structure here goes from George Kittle to Deebo to the return route by Jauan Jennings. Kittle is a matchup target here, and with nobody deeper than five yards at the snap, there’s no better time to throw the post to your freak TE.

But if he’s not going to, he needs to immediately get to his second look. The cluster creates a natural rub leaving Deebo wide open immediately out of his break.

But Purdy’s eyes never seem to get backside. It at least appears as though his eyes went to Christian McCaffrey, who also has a mismatch in man coverage and is tagged as the “Alert” in the play’s read structure. So he could have also been targeted.

But what we see next isn’t something we’d often see from Garoppolo — a throw made outside of structure. Purdy’s internal clock goes off, he bails, and he finds Deebo on the continuation of his route, with the linebacker bearing down on the scrambling QB, creating a window.

There is far more creativity in Purdy’s game, but this offense thrives within the confines of the offense. The 49ers are in the best spot they’ve ever been to compete without Garoppolo, but we must understand that success is far from guaranteed.

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