While J.J. Watt won the NFL’s defensive player of the year award, in our opinion, LA Rams Defensive Tackle Aaron Donald was the best player in the league during the 2015 season. Traditionally, defensive linemen have been judged on two statistics: sacks and tackles. In both categories, Watt got the best of Donald, but a deeper look at the numbers shows that Donald was more impressive.

Tracking QB disruption

While sacks are clearly important for a pass-rusher to get, that only happens on a very small percentage of pass-rush plays. A pass-rusher can disrupt the quarterback without sacking him, which is why we place such a premium on total pressure numbers. The extent to which a defense can pressure a quarterback has a huge impact on that quarterback's performance.

Consider these numbers from 2015:

  • If the defense fails to pressure the quarterback, then on average they have an NFL passer rating of 97.6

  • When a quarterback was hurried, that dropped to 78.4

  • When a quarterback was hit, that dropped even further to an NFL passer rating of 56.1

When you combine sacks and hits, Donald had 37, 14 more than any other 4-3 defensive tackle or 3-4 nose tackle. On a per-play basis, Donald was able to pressure quarterbacks on 15.8 percent of his pass rushes, which was the best for defensive tackles—and ahead of Watt’s 15.2 percent.

Affecting the ground game

Looking at sacks, hits and hurries accounts for the majority of players’ production in the pass game, so to isolate the run game we look at run stops and run stop percentage. While making a tackle is better than missing it, not all tackles are created equal. There is a difference between making a tackle where the offense doesn’t gain any yards compared to a tackle after the offense has already gained yards.

To fix this, we look at stops, which are tackles only on plays that constitute a loss for the offense. Donald and Watt both had 37 run stops, which was tied for the best for defensive linemen who aren’t nose tackles. Looking just on a per-play basis, Donald had a run stop on 10.9 percent of his run plays, which was the best for defensive tackles who aren’t nose tackles, and better than Watt’s 10.5 percent.

There are plays where a defender can disrupt a run play without being the one to make the stop, like beating a block to force the running back to make a cut, or standing up a lineman at the point of attack. By our grading, Donald had 71 such plays in 2015, while Watt has never had more than 49.

In both the run game and pass, Donald outperformed Watt. He was dominant in a way that no other player was in 2015, and stands as our top player of the 2015 season.