It took an unfortunate injury to Tyrod Taylor, but Justin Herbert took the NFL by storm last season, throwing his way into prominence after setting rookie records for touchdowns and completions while also putting up impressive yardage totals. But after such an explosive performance, it makes sense to ask: Can he keep it up? How likely is it that 2021 will be disappointing for him and Chargers fans?

Quarterbacks who have poor rookie years don’t all become busts, but QBs who have good rookie years are far more likely to be good quarterbacks in the long run. Even if Herbert “regresses,” there’s a good chance Chargers fans still will be happy with him.

In order to ask the question about whether or not Justin Herbert will regress, we need to figure out what that means. Herbert hit some impressive volume records as a rookie but also ranked fourth among rookie quarterbacks in attempts since 2010, making it a bit easier to hit the touchdown and completion marks he set.

What stats track better for quarterback quality and impact on winning? Things like efficiency and the ability to produce from play to play. Luckily for Herbert, he did well there, too, ranking 10th among 31 rookie QBs since 2008 with at least 300 passing attempts in era-adjusted yards per attempt. His adjusted net yards per attempt, a measure that incorporates sacks, touchdowns and interceptions, ranks him fifth in that group.

So why would we even consider the possibility that Herbert could take a step back?

(Just FYI, this isn’t new territory. Multiple analysts have looked into the numbers and asked the same questions — which The Athletic’s Daniel Popper explored a few months ago — and the outlook isn’t generally positive. We can take a look at many of the reasons for that and see how well they hold up.)

The first reason is completely external to Herbert’s play: a shift in head coach, offensive coordinator and scheme. Eric Eager, a data analyst for Pro Football Focus, looked over the history of quarterbacks switching coordinators in their second season, and the overall numbers look pretty bad. To some degree, however, that’s to be expected. Not many quarterbacks who had good rookie years saw their coordinators leave. When looking only at rookie quarterbacks with high PFF scores who saw their coordinators leave, the effect largely disappears.

In this case, new Chargers coordinator Joe Lombardi is well regarded as an offensive mind. But you never know how successful a new coordinator will be until you see the offense, and there’s much more room to fail than thrive. It’s a concern, one that’s magnified by a subpar performance by the Detroit Lions offense under Lombardi in 2014.

There are a few ways to construct a comparison case for Herbert. One is to look at high-performing rookies, like Herbert, and check out how well they fared in their second year in the league. Another is to take a look at rookies closest to Herbert’s rate numbers.

The issue here is that these are two completely different sets of quarterbacks, as Herbert’s plaudits are largely volumetric. The high-performing rookies who won in rate metrics generally regressed a bit but stayed above NFL average. Those closest to Herbert’s rate numbers largely stayed around NFL average but improved slightly.

We can split the difference and look at those who matched Herbert in era-adjusted volume and rate statistics — players whose yards per attempt (YPA) and adjusted net yards per attempt (ANYA) were near league average but whose passing yards per game were 10 percent higher than the league’s average. That leaves us with three quarterbacks: Andrew Luck, Cam Newton and Baker Mayfield.

That’s not a bad group to be compared to from a career perspective, but as second-year players, there wasn’t much immediate progression by the group. Newton did create additional production on the ground, but that doesn’t seem like a likely outcome for Herbert.

Given that we have a fairly small group of players to compare to Herbert’s overall passing profile, it might be better to look at what predicts future performance in general among quarterbacks, regardless of rookie status. The most common concern expressed for Herbert is his remarkable performance under pressure and his ability to sustain that into his second year.

That’s not a knock on Herbert. As Sam Monson of PFF puts it, “A quarterback playing lights out under pressure or in high-leverage situations may be justifiably praised for that performance, but it doesn’t necessarily mean he can repeat it in the future.” The research at Sports Info Solutions found the same thing, that performance under pressure can explain quarterback outcomes but doesn’t do a very good job providing information about future performance.

There are likely several reasons for this, and one is surely the lower number of snaps under pressure than snaps in a clean pocket. But that can’t be the only explanation because larger samples of pressured pocket performances demonstrate low stability when compared to smaller samples of clean-pocket performances.

One reason might be, as PFF’s Kevin Cole put it, clean pockets tend to be similar while each pressured pocket can be different in its own way. That could be one explanation for why performance under pressure isn’t that stable from season to season when compared to clean pockets — we’re including dissimilar situations, like interior pressure on a quick snap and edge pressure on a long bootleg.