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Bill Wippert/Associated Press

The 49ers needed to make a change at cornerback. Tramaine Brock was burned for five game-changing plays in the last three weeks, prompting 49ers defensive coordinator Jim O'Neil to roll the dice on third-year cornerback Keith Reaser. He played a lot better, recording three tackles and one pass breakup.

The 49ers' best rookie has been cornerback Rashard Robinson, and again, he came up huge for the San Francisco defense. He played everything in front of him, didn't get beat and made plays on the ball. The rookie was tested early but not often, as Bills quarterback Tyrod Taylor found out quickly that the LSU product is poised beyond his years at the cornerback position.

Eric Reid has been a liability in pass coverage all season. Just look back to the Seattle Seahawks game, when Reid gave up a huge pass play to Seahawks tight end Jimmy Graham, if you need an example.

Reid struggled once again, giving up a 30-yard touchdown to 2013 second-round pick Justin Hunter. The Tennessee product beat Reid on a simple post route. This is by no means a fixable problem. Just one drive later, Reid was hit with a pass interference call McCoy in the end zone.

Grade: D