Mario Williams had sacks in 7 out of 16 games. Compared to the guys above, I'd say he was lacking. Everyone else was more consistent, other than Charles Johnson. I'm pretty sure Mario makes the most as well. So I think there's reason to think that he disappointed compared to expectations.
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Now, just because he was lacking compared to the above, or maybe even my own expectations... he's still our best DE, and he's still a good player.