Top offensive matchup nightmares

— -- NFL coaches don't sleep much during the season.

Between film study, game plans, practice and everything else that goes into preparation for Sundays, shut-eye is hard to come by.

It's especially difficult when preparing for certain star players; the kind of rare physical specimens who can leave a coach scrambling for answers before, during and even after the game. With that in mind, let's begin part one of a two-part feature examining the 10 most nightmarish matchups on either side of the ball in the NFL, based largely off of suggestions from NFL sources around the league.

To be clear, these players are nightmarish because of uncommon physical traits. While there is no quarterback better than Peyton Manning or Tom Brady, defenses that can apply pressure and impact the integrity of the pocket have a chance to slow them down because of their limited mobility. Just one quarterback cracked our list, and he just so happens to have the movement skills of a running back and that applies a different type of stress on a defense that Brady and Manning aren't capable of.

Here are the top 10 offensive nightmare matchups in the NFL:

1. Calvin Johnson, WR, Detroit Lions

The most physically dominant offensive player regardless of position, Johnson is an impossible cover and was the most popular response among coaches and scouts consulted. He's built like a tight end at 6-foot-5 and nearly 240 pounds but maintains the track speed he came into the league with. He can stretch the field vertically, win with his route-running skills on the intermediate level and terrorize defenses as a run-after-catch threat on wide receiver screens.

Even as the Lions have gone through a roulette of complementary receivers to try and take pressure off of Johnson, Megatron has breezed past, through and around opposing defenses. He's a one-man wrecking crew; there's simply no slowing down the game's best receiver.