Bill Belichick is back at it again.

After the flurry of trades earlier in the offseason, the New England Patriots extended an offer sheet to Buffalo Bills running back Mike Gillislee. The offer is for a total of $6.4 million over two years, with $4 million of that in the first year, per NFL Network’s Ian Rapoport. The Bills now have five days to match the offer, but there’s already been indications they won’t.

There's a lot to take in here, even though it's a seemingly simple move. So, let's dive into all the aspects surrounding this offer.

Why Would New England Want Gillislee?

Before this offer, New England had $8 million of their cap dedicated to running backs, the 12th-highest figure in the league per Over The Cap. That includes Dion Lewis, James White, and the $3.15 million given to Rex Burkhead this offseason as a free agent. The Patriots appear to have the backfield covered.

But Gillislee has been really good -- like the most efficient running back according to our Net Expected Points (NEP) good.

There were 42 running backs who carried the ball at least 100 times last year. Gillislee's Rushing NEP per attempt was 0.30, and his Success Rate (the percentage of carries that positively impact NEP) was 57.4 percent. Both of those numbers led the league. Lesean McCoy had the second-best Rushing NEP per attempt (0.16), while Bilal Powell of the New York Jets posted the second-best Success Rate (49.6 percent).

To put in perspective as to just how efficient he was last year, consider this -- there was a bigger gap between Gillislee and McCoy in Rushing NEP per attempt than there was between McCoy and the 17th-ranked running back last season. With regard to Success Rate, there was a bigger gap between Gillislee and Powell than between Powell and the 16th-ranked back.

That is rather impressive.

This isn’t really a one-year aberration for Gillislee, either. He played a much smaller role in the 2015 offense, but was also quite good. He had 47 carries that season, which puts him in a group of 82 running backs who had at least 40 carries in 2015, and he ranked 10th in Rushing NEP per attempt.

How Would Gillislee Fit With the Patriots?

Gillislee, who was listed at 219 pounds on the Bills' official website, would be the biggest back on New England's roster -- just above Burkhead. While he isn’t nearly the big power back LeGarrette Blount was, he was nearly an unstoppable force near the goal line. Seven of his eight touchdowns came on 10 carries within five yards of the end zone, which was good for a 90 percent Success Rate and a 1.28 Rushing NEP per attempt.

Blount saw 29 carries in the same situation last season, scoring 13 touchdowns on a Success rate of 51.72, but he produced a Rushing NEP per attempt of just 0.25.

It's likely that he ends up taking over some of Blount’s role as the heavy back. However, Blount did get 299 rushing attempts last season and there’s little chance Gillislee’s workload increases that much. Burkhead, a much better inside runner than most give him credit for, is also likely to take some of that responsibility.

With Burkhead and Gillislee in the same backfield rotation, the Patriots will show a clear preference for efficiency.

There were 60 running backs who had 70-plus carries last season. Among them, Burkhead ranks third in Rushing NEP per attempt (0.15), and he was the only rusher besides Gillislee to have a Success rate above 50 percent (54.1).

Belichick can publicly say he doesn’t look at analytics websites, but this type of information is not passing him by. It’s not a coincidence that the Patriots have targeted both Burkhead and Gillislee this offseason.

While White and Lewis are threats in their own right, White is much more of a receiving back (83 targets to 39 carries in 2016) and Lewis is still an injury risk after playing in 14 games over the past two seasons. Having all these backs in one rotation also doesn’t preclude them from being on the field at the same time. Per the Sharp Football Stats personnel frequency tracker, no team ran more plays with at least two running backs on the field last season than the Patriots (399 total plays).

What Are the Bills Doing?

If Buffalo indeed doesn't match the offer sheet, it would be the second time in two years the Patriots stole a restricted free agent from their AFC East opponent, with wide receiver Chris Hogan being the first. The Bills didn't get anything in return last year, though, since Hogan went undrafted.

Gillislee was drafted in the fifth round, and it's likely the Bills thought most teams would rather use that pick on a rookie back in what appears to be a deep class at the position instead of chasing after a restricted free agent. The Patriots, of course, are not most teams.

For less than $1 million, the Bills could have used a second-round tender on Gillislee and this offer sheet probably doesn’t happen -- a second-round tender was worth $2.746 million this offseason, while an original-round tender (what Buffalo extended to Gillislee) was worth $1.797 million.

The Bills now might need to dive into this deep class at running back to find a replacement since they just have Jonathan Williams, last year’s fifth-round pick who had only 27 attempts last season, as a backup. Buffalo had the second-best running game in 2016 when looking at Adjusted Rushing NEP per play, barely trailing the Dallas Cowboys. They were the second-most run-heavy offense in football, and they're going to need someone to spell McCoy on occasion. It'll be tough to find someone as good as Gillislee to do it, though.

For the second year in a row, Buffalo stands to lose an important role player due to a cheap and lazy gamble by the front office. And for the second year in a row, the Bills’ loss is the Patriots’ gain.