Last season was difficult for the NFL's highest-paid running backs. Dalvin Cook and Nick Chubb remained extremely effective when healthy, Austin Ekeler took over as a two-way force and Jonathan Taylor emerged as the league's best young back, but the players who expected to be elite weren't at that level.

Let's break down what happened in 2021 with four of the most prominent backs -- Christian McCaffrey, Derrick Henry, Alvin Kamara and Ezekiel Elliott -- to get a sense of what they might do this season. Injuries played a role and could do so again, but in several cases, I found the stories most often being told about these four masked more significant issues. In other cases, I found the popular perception to be totally true.

I'll run through all four using advanced metrics from ESPN Stats & Information and NFL Next Gen Stats and then project what an average season might look like for them in 2022. I'll begin with Carolina, where the consensus No. 1 fantasy football pick in 2020 and 2021 has spent most of the past two years on the sideline.

Christian McCaffrey, Carolina Panthers

  • 2021 stats: 99 carries for 442 yards and 1 TD; 37 catches (41 targets) for 343 yards and 1 TD

Let's start with the league's most expensive back. When McCaffrey signed his four-year, $64 million extension in 2020, he was the exception to arguments about signing running backs to extensions.

For one, his prodigious receiving numbers gave him a wider range of values than the typical back. More importantly, perhaps, his medical record was pristine: He hadn't missed an NFL game while lining up for more than 90% of the Panthers' offensive snaps in back-to-back seasons.

Since then, McCaffrey has played only about 21% of Carolina's offensive snaps, missing 17 of 27 possible games with ankle, shoulder and hamstring injuries. The same player who ran for 100 or more yards six times across the first nine games of the 2019 season hasn't hit that mark in a single game since. Teams reportedly called the Panthers to trade for McCaffrey this offseason, but with Carolina looking for a first-round pick and a young player, no deal was reached.

After seeing McCaffrey spend most of the past two campaigns on the sideline, my first instinct was to wonder whether the 25-year-old was likely to be back on the field more often in 2022. He is expected to hit training camp at 100 percent, but the same thing was true last year and he was then injured halfway through Week 3.

How often does a running back struggle through two injury-riddled years and make it back to his previous productivity?

I looked at every back since the merger who had 300 touches in back-to-back seasons, a mark McCaffrey topped comfortably in both 2018 (326 touches) and 2019 (403). Then I focused on the players who followed those two big seasons by failing to top 300 touches combined over the two ensuing seasons. McCaffrey had 76 touches in 2020 and 136 a year ago for a total of 212 over the past two campaigns. Did those guys return to their starting jobs and play at a high level?

Not really, no. Many of the players who had this happen retired, including legends Curtis Martin, Walter Payton and Ricky Watters, the latter of whom might be the closest comp to McCaffrey. Those players were older, but guys who had this happen in their 20s also didn't get back on track. (I'm leaving aside Travis Henry and Ray Rice, who had their careers impacted by off-field behavior.) Terrell Davis came back and played one more half-season as a starter before retiring.

There is no back who went through this sort of dramatic swing as early in his career as McCaffrey has for the Panthers, with a few borderline exceptions around holdouts. Errict Rhett had two competent seasons to begin his career with Tampa Bay, held out and never established his prior level of performance. Bobby Humphrey (father of current Raven Marlon Humphrey) was a star for the Broncos in 1989 and 1990, held out for most of the 1991 season, was traded to the Dolphins before 1992 and was out of football for good the next season, in part because he had been shot in the leg in early 1993. McCaffrey was arguably the NFL's best back at ages 22 and 23, didn't hold out, and was a part-time player at 24 and 25.