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Stud: Ahmad Brooks
Ahmad Brooks has seven hurries, 2.5 sacks and 25 tackles. He picked up some of the pass-rushing slack against the St. Louis Rams from the absence of Aldon Smith, whom I considered for both the stud and dud categories before ultimately leaving him off this list.
Dud: Nnamdi Asomugha
Call this the dishonorable mention. Nobody was expecting Nnamdi Asomugha to be the shutdown corner of yore, but the 49ers weren't expecting awful play either. Pro Football Focus ranks Asomugha 134th out of 142 cornerbacks (subscription required). That was based on his performance in San Francisco's first three games. He was out with a knee injury for the fourth.
Biggest Surprise: Donte Whitner
Donte Whitner's PFF coverage grade is plus-4.2 (subscription required), third best in the league. Sure, he dropped a potential interception against the Rams, but he also had a diving interception (shown above) later in the game. After being a coverage liability last year, Whitner has shown so far this season that he can be trusted.