There's a common phrase that gets tossed around in the NFL: Next man up.

With the physical demands of the sport, back up players are often thrust into prominent roles when starters inevitably go down to injury.

No place has been more emblamatic of this than in Arizona, where it seems as if on a weekly basis players often step up to cover injuries to their teammates. In the passing game, Michael Floyd and John Brown have found themselves filling in for each other admirably whenever injuries have forced the other out of games. 

And now with the Cardinals leading rusher, Chris Johnson, out for the rest of the regular season with a broken leg, the healthy players remaining in the backfield are once again being asked to step up to fill the void.

With the Cardinals offense performing well without the veteran lead back so far -- with rookie David Johnson totaling 120 yards and a touchdown on 24 touches in a 27-3 victory over their division rivals, the St. Louis Rams, last week -- the question is: will they be able to keep this up?

The Chris Johnson Revival

Prior to his regular season-ending injury, Chris Johnson was having his best season in years. His 196 carries -- which is still the fifth most among all tailbacks this year -- resulted in 814 yards and 3 rushing scores over the span of games. And even after missing part of Week 12 and all of Week 13 with a broken leg, Johnson's rushing yards total still ranks as the sixth leading rusher on the year.

That's quite a contribution to the Cardinals offense.

However, a closer look at the numbers reveals that Chris Johnson hasn't been the most efficient of tailbacks. Among the top five tailbacks in carries this year, Chris Johnson's -0.07 Rushing lack of trust in rookies by head coach Bruce Arians led to David Johnson falling to third on the depth chart in Arizona, despite his relative success early on -- in his first two games as a pro, even though he played just 21 total snaps, Johnson still found a way to record a rushing, receiving, and kickoff return touchdown over that same timespan.

And as the season has progressed, Johnson continued to produce on limited touches. A testament to David Johnson's ability to pick up chunks of yards both on the ground and through the air, the Cardinals rookie not only owns an above-average 0.03 Rushing NEP per attempt, but also owns a Reception NEP per target of 0.81 to lead all tailbacks with at least 30 targets on the year.

These stellar averages are also thanks in large part to his stellar track record in the scoring department. Through the first 12 games of the season, Johnson has amassed eight touchdowns (nine if you count his kickoff return touchdown in Week 2) on just 78 total touches.

And as David Johnson has shown, he's more than capable of continuing this level of productivity with a scaled-up workload. Receiving the start last week against a Rams defense that ranked fourth against the rush going into Week 13, David Johnson rushed 22 times for 99 yards and added two receptions for 21 yards and a touchdown.

Perhaps the biggest indicator that the Cardinals running game is in good hands with the rookie -- albeit in a small sample size -- Arizona actually ran a greater percentage of run plays last week against the Rams than in 10 full games with Chris Johnson as the starter; through Weeks 1-11 the Cardinals boasted a pass-to-run ratio of 1.28, while last week with David Johnson as the lead back the Cardinals called a pass-to-run ratio of 1.19 as they cruised to a 27-3 victory.

Leaning on the Passing Game

Now should the team's rookie tailback falter, the Cardinals have one hell of a backup plan in Carson Palmer. Playing once again at a Pro Bowl-caliber level, Palmer has thrown for 3,693 yards and 29 touchdowns so far this year, and is actually second in the league in passing efficiency according to our metrics with a Passing NEP per drop back of 0.34.

These impressive numbers are thanks in large part to perhaps one of the most formidable wide receiver corps in the league. With a receiving quadruplet of Larry Fitzgerald, John Brown, Michael Floyd, and J.J. Nelson, opposing defenses have been stretched thin trying to contain this group of pass-catchers.

While Fitzgerald's 0.68 Reception NEP per target ranks him in the top 35 in efficiency among all wideouts with at least 50 targets, Brown's 0.90 and Floyd's 0.89 marks actually rank them fifth and seventh in the league, respectively. And while Nelson has only seen 22 targets on the year as the team's fourth receiver, he owns an incredible 1.40 Reception NEP per target himself.

It's clear that not only will this passing offense keep opposing defenses honest, but if the Cardinals begin to struggle in the ground game, the passing game can more than adequately make up for it.

Conclusions

Despite the loss of Chris Johnson and his production for Arizona in their rushing attack, it's obvious that with a more than capable fill-in in David Johnson. With David showing better production on a per-touch basis than Chris, an argument could be made that perhaps the Cardinals running game has improved with the reins turned over to the rookie.

And with a formidable passing attack that features Palmer, Fitzgerald, Brown, and Floyd to compliment the ground game, the Cardinals offense should continue to roll against opposing defenses even in the absence of their veteran tailback.