If I asked you to list the most efficient running backs this season, chances are Mike Gillislee's name would not come up. If I expanded that question to the last 16 years, it's virtually certain it won't.

No one seems to be talking about the Buffalo Bills' second-stringer at all. But they should be; he's earned it.

Here's why.

Gillislee's Season

When evaluating efficiency, we have to move past the raw numbers like total yards -- those can be products of volume. Instead, we require a metric that measures the average gain on any given play.

For running backs, yards per carry is a seemingly logical place to start measuring efficiency. However, while it has some value, it leaves much to be desired. Instead of relying solely upon a simple statistic like yards per carry, we at numberFire use a metric called Net Expected Points (NEP).

NEP factors the value of down and distance as well as standard box score information to measure a player's impact on his team's scoring expectation. It's a cumulative statistic, so to measure efficiency, we need to look at Net Expected Points per play. You can learn more about NEP in our glossary.

Despite all of that -- let's start simple and see how Gillislee fares by traditional measures.

The table below shows the top 10 backs by yards per carry with a minimum of 75 rush attempts (statistics via Pro Football Reference).

Even by superficial metrics, Gillislee excelled this season. His 5.71 yards per carry trailed only Ty Montgomery and Jalen Richard and led teammate LeSean McCoy. In fact, Gillislee's yards per carry ranked 14th-best since 2000.

When it comes to scoring, only McCoy and Ezekiel Elliott scored more touchdowns among the top 10 -- and they had more than two to three times as many carries.

In fact, among backs with fewer than 200 carries, only Latavius Murray had more touchdowns; he scored 12 in 195 attempts, but his yards per carry wasn't high enough to make the top 10. He averaged 4.04 yards per carry, only good enough for 30th out of 56 qualifying running backs.

That illustrates one of the problems with using yards per carry to measure efficiency: it doesn't factor in touchdowns. Hide that column in the above chart and it would be easy to say that Jalen Richard and Ty Montgomery had more efficient seasons than Gillislee. Obviously, touchdowns matter.

Advanced Metrics

Richard and Montgomery may have the lead in yards per carry, but both Rushing NEP and Rushing NEP per carry tell a different story. The gap between them and Mike Gillislee becomes even clearer when factoring Success Rate, or the percentage of plays that contribute positive NEP.

Let's look again at the most efficient running backs (minimum 75 carries), this time by Rushing NEP per play.

For reference, the average Rushing NEP per play for running backs this season was -0.02. In other words, the average back was losing his team 0.02 points every time he carried the ball.

Wow. Mike Gillislee.

His numbers make even the closest rivals look silly. His 0.30 Rushing NEP per play nearly doubles the second place mark, set by his backfield mate.

In 133 more carries, McCoy added only 6.16 more Rushing NEP than Gillislee did -- and Ezekiel Elliott only added 5.26 extra Rushing NEP in 220 more carries.

Gillislee's 57.43% Success Rate -- also league-leading -- explains some of how he managed that. It means that on 58 of his 101 rushing attempts he contributed positive NEP, a rate that is practically unheard of. League average for running backs this season was 40.28%.

Bilal Powell posted the second-best mark with 49.62%, nearly eight percentage points lower than Gillislee, yet still an excellent Success Rate.

"Touchdown Mike"

I found Gillislee's excellent numbers shocking. I knew when I started my research that his season had been excellent, but I was not prepared for just how impressive it was. So I set out to poke holes in it. Minor spoiler, though I suspect you've already guessed it: I failed.

But I did learn some interesting things in the process. As his nickname implies, Mike Gillislee received a billing as the Buffalo Bills' red zone back. To be fair, it did seem like he got more than his share of high-value opportunities.

But like many things coaches say, the reputation as a red zone back they gave Gillislee turned out not to be true. He carried the ball a total of 18 times in the red zone, a team market share of 17.8% -- slightly more than McCoy's red zone carry percentage of 16.2%.

However, red zone rushes did in fact have a higher average NEP this season: 0.04, or 0.06 greater than the overall average of -0.02. But Gillislee's red zone efficiency was otherworldly. He posted a staggering 0.93 Rushing NEP per play in the red zone, with an equally staggering (and unsustainable) 83.33% Success Rate. He clearly did bolster his efficiency with his red zone carries, but here's the catch.

His Rushing NEP per play outside the red zone was 0.15 with a 49.4% Success Rate, still good enough to rank second, after only McCoy. And that includes McCoy's 38 red zone carries.

In other words, his red zone efficiency may be unsustainable, but he still ran with impressive efficiency outside of it.

A Place in History?

Since 2000, the extent of our database for NEP, no running back with at least 75 carries has recorded a better Rushing NEP per play than Mike Gillislee in 2016. The table below compares his numbers to the other backs in the top five by Rushing NEP per play.

Normally, we round to two decimals, but to show the difference between Gillislee and Moe Williams, I went to three. It's a small difference, but it does distinguish Mike Gillislee as having the best overall Rushing NEP per play in this subset.

It's an interesting list. Karlos Williams ranking fourth raises a new question: what is the significance of the Bills' number two running back performing at record levels two years in a row? Obviously the Bills run the ball well; it's central to their offense, which can't hurt. But both Gillislee and Williams outperformed McCoy.

Sample sizes are too small to be definitive either way, but I can't find a reason why the Bills' change of pace back would be at a significant advantage over any other back, given similar opportunity. It's entirely possible that both Gillislee and Williams are simply good.

Though his touchdown rate is sure to regress, Gillislee definitely possesses enough talent to continue producing if he gets the chance. He's a name to watch over the offseason. He makes for an intriguing best ball pick and could be a solid ZeroRB pick in redraft leagues, depending on how the situation shakes out in Buffalo with their new coach.

At worst, he'll be one of the most valuable handcuffs come next season.