The 2016 season has not been kind to Arizona Cardinals quarterback Carson Palmer.

Palmer’s completion, touchdown, and interception rates are below the league average, he suffered a concussion on Sunday that puts his status for Thursday’s game in doubt. To top it off, Arizona is just 1-3.

The rough start comes on the heels of a career year for Palmer in 2015, when he threw for more than 4,600 yards and 35 touchdowns, while leading the league in net yards per pass and Passing Net Expected Points (you can read more about NEP here).

When viewed in this light, his current struggles are surprising, but should we have seen them coming?

Palmer's 2015

It seems easy to ask this now, but if Palmer turns things around, this success would buck a trend for aging quarterbacks, even good ones.

During his age-36 season last year, Palmer set career highs in raw yards per attempt, net yards per attempt, and adjusted yards per attempt both in standard form and relative to league average. His 6.5% touchdown rate was also the best of his career (and second-best of his career relative to league average).

In terms of advanced stats, Palmer also set career bests in terms of Passing NEP and Passing NEP per drop back. His 52.9% Passing Success Rate was the second-best mark of his career, narrowly trailing his 53.1% rate from 2005 (Success Rate is the percentage of plays that yield positive NEP).

All this success at an old age (for a football player, at least), put Palmer in rare company, as his 2015 season marked the 47th time a quarterback threw at least 200 passes and posted an adjusted yards per attempt rate that was at least a standard deviation above league average, according to Pro Football Reference (there have been 171 total seasons in which a quarterback who is 35 or older has attempted at least 200 passes).

Adjusted yards per attempt takes raw yards per attempt and gives a 20-yard bonus for touchdown passes and a 45-yard penalty for interceptions (it is inferior to adjusted net yards per pass because it does not include sack yardage; however, sacks were not tracked until the 1970s, so AY/A is useful here because it allows us to look at all of NFL history).

Before we go on, make sure you’re paying attention New England Patriots fans -- because Tom Brady joined this group last season as well.

The Following Year

37 of these quarterbacks returned the following year to throw at least 140 passes, and things generally did not go well for this group. In the seasons in which they were at least a standard deviation better than the mean, this group had an average AY/A+ of 121.9; the following year, their average AY/A+ fell to 105.4 (AY/A+ is adjusted net yards per pass relative to league average, in which 100 is always average and 15 units equal one standard deviation above or below the mean).

Only eight of these quarterbacks saw their AY/A+ increase, while 19 saw a decline of more than one standard deviation, and nine fell by at least two. Peyton Manning (last year) and Y.A. Tittle (1964) fell by more than three standard deviations, suggesting all-time greats are not exceptions here.

Here is the group:

The picture may be even bleaker for Palmer though. We can subdivide this group of 37 even further, by looking at the eight quarterbacks who (like Palmer) had a career year in terms of AY/A+ and returned to play the following season (note that Vinny Testaverde, John Elway, and Jim Plunkett also had their best AY/A+ seasons when they over 35 but did not get significant playing time the following year; Elway and Plunkett retired, and Testaverde missed virtually all of 1999 with an injury).

This group did better than the population as a whole (average AY/A+ of 128.4) but also saw a steeper decline the following season (average AY/A+ of 92.9, meaning they actually played at a below league-average level).

Except for Doug Flutie (who declined by 25 units or about 1.7 standard deviations), this group all declined by at least two standard deviations. Only Randall Cunningham (in 1999) and Steve DeBerg (1991) played at an above league-average level.

None of this is to suggest Palmer cannot play at a high level going forward. The Cardinals' offense remains loaded with talent and is led by Bruce Arians -- widely considered one of the league's better coaches -- and Palmer himself has been an above-average quarterback for most of his career.

It does seem, however, that history is not on his side.