Not too long ago, Rex Ryan and Jeff Fisher were two of the most successful and well-respected head coaches in the NFL. By the end of the 2016 season, both had squandered all of that goodwill after turning talented rosters in Los Angeles and Buffalo into mediocre teams.

The Bills and Rams came to their senses at the end of last season and canned their respective coaches. But firing a bad coach only matters if the move is followed up with the hiring of a good on. Based on the early returns, both Buffalo and Los Angeles did that this offseason and now both are sitting atop their divisions at 3-1 after pulling upset wins in Week 4.

That the Bills and Rams are enjoying success is somehow both surprising and unsurprising. I praised the Bills for the Sean McDermott hiring when the move was announced. I also picked the Rams as my playoff sleeper back in August because of their hiring of Sean McVay. But even as someone who was high on both coaches, I did not expect the two teams to be this good this soon. I underestimated the impact of replacing a bad coach with a good one.

Let’s take a look at both teams and figure out how their first-year coaches turned them around and if we should expect their hot starts to turn into special seasons.

Buffalo Bills

McDermott’s best quality, and one he shares with the best coaches in the league, is his adaptability. The progression of his defensive scheme in Carolina was impressive to watch and the biggest reason I believed he’d be a great head coach in Buffalo. Here’s what I wrote about him back in January when his hiring was announced:

The Panthers’ defensive scheme was not overly complicated at the beginning of McDermott’s tenure. Out of necessity, the Panthers ran conservative zone coverages behind a four-man rush. The secondary was among the worst in the league and the team had a strong defensive line … As Luke Kuechly and Thomas Davis emerged as the best linebacker pair in the league, the defensive identity began to shift. Carolina was still playing those basic zone defenses, but with two smart, ball-hawking linebackers, they could mix things up a little more … When Josh Norman broke out as one of the league’s best corners, Carolina expanded their defensive repertoire even more. What was once the most predictable defenses in the league had become one of its most varied: The Panthers played Cover 3, they played Quarters, they played man coverage. They sent fire zone blitzes. They blitzed from double-A gap looks.

McDermott played to his personnel’s strengths with the Panthers. Now he’s doing it in Buffalo and the results are pretty much the same. Buffalo will mix in shrewdly designed zone blitzes and some man coverage when the situation calls for it, but, for the most part, McDermott has prefered to play conservative zone coverages. It’s very much a bend-but-don’t-break defense and the Bills have done very little breaking thus far.

One thing that has stood out early in the season, and especially so during the Bills’ upset 23-17 win over the Falcons, is that this defense just doesn’t make a lot of mistakes. That forces offense to string together long drives in order to put up points, and only a handful of teams are capable of doing so.

The opposite was true during the Rex Ryan era. The boisterous coach inherited a good defense that was built around a dominant 4-3 front that could get after the quarterback with a standard four-man rush. So what did Ryan do? He forced his hybrid blitzing scheme on the Bills, which minimized the benefits of having a great line and put more pressure on an underwhelming secondary. The result? A 15-16 record over two seasons and two defenses that ranked 19th in yards allowed.

The Bills underachieved under Ryan because he refused to adapt. Now they are overachieving thanks to the pliability of McDermott. But will it continue? I’m not so sure it will. This team is going to remain competitive throughout the season, but there just isn’t enough talent on this defense right now to keep this up. The Bills need to improve at the linebacker position if McDermott’s defense is going to function at a high level over the course of a season.

McDermott is building something special in Buffalo, but Bills fans may have to wait a season or two before this team is ready to compete with the Patriots in the AFC East.

Los Angeles Rams

Jared Goff never had a chance in his rookie season. Not in the Rams’ dated offensive scheme. Los Angeles’ offense was a chore to watch in 2016. It just didn’t have the talent on the offensive line to run the scheme Fisher wanted and Goff was horribly miscast in the run-heavy, deep play-action passing system after having played in a pure spread at Cal.

Enter McVay, who had just spent the last few seasons making Kirk Cousins look like a top-10 quarterback thanks to brilliant play designs that make things so much easier on a quarterback. All of a sudden, Goff has bounced back from a historically bad rookie season and looks like a top-10 quarterback himself after going on the road and beating the Cowboys.

The only player who is benefitting more than Goff is from the McVay hire is Todd Gurley, who looks more like the player we saw back in 2015. Gurley is no longer being used as a battering ram like he was far too often last season. McVay has found different way of getting the running back the ball in space where he can use his speed to torch defenses.

That includes a bigger role in the Rams passing game. Whereas last year Gurley was used as a blocker, he’s now one of Goff’s favorite targets. Gurley has already caught 20 passes over the first month of the season. He caught 43 all of 2016.

And Gurley isn’t just catching screens and passes out in the flats. McVay has turned him into a legit downfield threat. His 53-yard touchdown catch came on a play design McVay wisely borrowed from Andy Reid.

NFL game-planning is all about exploiting mismatches and that’s exactly what the Rams did here. Los Angeles was able to isolate Gurley on a linebacker who had no chance of sticking with him — especially not with the fake to Tavon Austin drawing the second-level defenders to the line of scrimmage. And once Gurley gets his hands on the ball in the open field, it’s pretty much over for the defense.

As for what to expect from this team going forward, the Rams’ outlook is a bit brighter than Buffalo’s. GM Les Snead did a fantastic job of building around Goff this offseason, adding a legit blindside protector in Andrew Whitworth and three reliable receivers in Sammy Watkins, Cooper Kupp and Robert Woods. This offense, which had been punchless for so long, is all of a sudden stacked. And the defense, which has always been talented, finally has a smart defensive mind leading it in Wade Phillips, whose second-half adjustments helped the Rams pull off the upset in Dallas.

The Rams are for real. They’ve had this in them for a few years now, but they finally have the coaches in McVay and Phillips to unlock that potential.