The NFL is a passing league. You know this. I know this. Just about everyone in the league knows this. 

Because it's a passing league, quarterbacks are the most important players on the field. And just about every quarterback has that one pass-catcher he trusts more than anybody else -- the guy he wants to go to in key spots, with the game on the line. With that in mind, we're going to use this space over the next several weeks to dig into some of the league's best passing-game combinations and what makes them tick. 

We began last by exploring the connection between Dak Prescott and Amari Cooper, which is defined by how each player has raised the other's game since they came together. Below, we'll delve into Deshaun Watson and DeAndre Hopkins, whose partnership is defined by degree of difficulty. 

We have to start with Hopkins, because the degree of difficulty he faced before hooking up with Watson is where this story really begins. Before Watson took over the starting job, Hopkins played with the following quarterbacks, listed in order of the number of passes they threw in his direction: Brock Osweiler, Brian Hoyer, Tom Savage, Ryan Fitzpatrick, Ryan Mallett, Case Keenum, Matt Schaub, T.J. Yates, and Brandon Weeden. Yes, that list is exactly as horrifying as it looks. 

Combined, that group of players completed 1,367 of 2,314 passes thrown during the pre-Watson portion of Hopkins' career, for 15,097 yards, 83 touchdowns, and 63 interceptions. That's a 59.1 percent completion rate, an average of 6.52 yards per attempt, and a 79.1 passer rating. And that's during a four-season span where the league average completion percentage was 62.5, the league average yards per attempt was 6.75, and the league average passer rating was 86.8. 

This was a group of quarterbacks who were considerably below-average across the board. And yet, during that four-year span, Hopkins ranked 12th in the NFL in receptions (317), fourth in receiving yards (4,487), and 20th in receiving touchdowns (23). Among the 31 players with at least 250 receptions during that time, his 14.15 yards per reception average ranked eighth-best. And this was all before he played with the only good quarterback with whom he has played in his career. It's no wonder that since Watson took over, Hopkins ranks second in receptions (286), third in receiving yards (3,695), and first in receiving touchdowns (28). 

Since arriving in Houston, Watson, too, has been given a high degree of difficulty. During his three years in the league, he has been saddled with an organization that seemingly has not cared all that much about making sure he is well-protected -- fielding offensive lines that were among the league's worst in each of those seasons, and employing a coach who cares not for adjusting his play-calling to counteract that weakness. 

Watson was the single most pressured quarterback in the NFL during his rookie season, according to Pro Football Focus. (Lest you think that blame all belongs on his shoulders for holding the ball too long, Tom Savage ranked second, with a nearly identical pressure rate.) His sophomore season was an improvement, as he was merely the second-most pressured passer in the league. The only guy ahead of him was Jimmy Garoppolo, though, and he tore his ACL in the third game of the season. This year, Watson ranks seventh in pressure rate, though a couple of the players ahead of him (Sam Darnold and Josh Rosen) have not played full seasons. 

Despite all that pressure, Watson has blossomed into a full-blown star -- and this season, an MVP candidate. He's completed 67.4 percent of his career passes at an average of 8.1 yards per attempt, throwing 63 touchdowns against just 23 interceptions and accumulating a 103.1 passer rating along the way. Among the 36 players who have thrown at least 500 passes since Watson entered the league, his completion rate ranks fourth, his yards per attempt average second, his touchdown rate third, and his passer rating fourth. 

And all of that is before we get to the enormous value he adds with his legs, both in terms of creating plays under pressure and in actually running the ball himself. Watson has run for 1,111 yards and 12 touchdowns in just 33 career games. His career averages of 5.8 carries for 33.7 yards essentially make him a change-of-pace back in addition to a quarterback.

Once they came together, Watson and Hopkins excelled at making high degree of difficulty plays. Take this touchdown against the Chiefs from Watson's rookie year, for example. 

Watson takes a couple steps back to keep the pressure out of his face, then fires an absolute later 40-plus yards downfield and into the end zone. He fits the ball into a window between two closing defenders, and Hopkins just goes right up and beats both of them to the ball. It's ridiculous. 

Check out these two sideline throws from Watson to Hopkins, both of which are dropped in a bucket, with the quarterback having put the ball not only in a place where only his receiver could catch it, but also arguably in the only place it could have been caught at all. 

They've excelled throughout their time together at working in tight spaces, especially on throws over the middle of the field. Watson has quickly become an extremely accurate passer on these types of throws, while Hopkins' combination of catch radius, body control, and incredible hands make him one of the NFL's best receivers at hauling in contested catches. He knows exactly how to use his body to shield off a defender, whether it's in the end zone, across the middle, or as he's about to get hit. 

This year, it seems like the Texans have gone to considerable pains to lower the degree of difficulty on Watson-to-Hopkins throws. Hopkins' catch rate sits at a career-high 72.1 percent, but his yards per reception average of 9.9 is by far the lowest of his career. During the past two seasons, Hopkins was targeted on 67 throws at least 20 yards in the air, catching 28 of them for 855 yards and eight touchdowns, per Pro Football Focus. This season, he has been targeted deep downfield only 11 times, catching just two of them for 50 yards and zero scores. 

It's good that the Texans have been trying to make life easier for these guys, but they have clearly taken things a bit too far at times, and not just with the near-elimination of the deep portion of the field. Hopkins was essentially used as a running back against the Chiefs earlier this year. These are his nine catches from that game, which totaled just 55 yards.

That's a waste of one of the NFL's best wideouts. The Texans still managed to walk away from that game with a win thanks to the combination of Watson's brilliance and strong running from Carlos Hyde, but they have gone just 2-2 in their four games since, and now face a somewhat difficult road as they make a push toward a second consecutive playoff berth. 

The Texans welcome the Colts to NRG Stadium on Thursday night, and it's a game they really need to win because they run up against the Patriots next week. Their season then ends with games against the Broncos, Titans, Buccaneers, and Titans again. Sitting at 6-4, the Texans cannot afford any worse than a 3-3 split if they want to win the AFC South. They might need better than that. When it comes down to it, they'll likely need Watson and Hopkins to bail them out of one more difficult situation.