Roster breakdown: DBs

SFI breaks down the 49ers' final 53-man roster by unit this week as the team closes in on Sunday's season opener against Atlanta. Today: Defensive backs

Starters: Ahmed Plummer (left cornerback), Mike Rumph (right cornerback), Tony Parrish (strong safety), Ronnie Heard (free safety)

Backups: Jimmy Williams, Keith Lewis, Shawntae Spencer, Dwaine Carpenter, Joselio Hanson

Practice squad: Mike Adams, Rayshun Reed, Richard Yancy

What's new: Fifth-year veteran Ronnie Heard takes over for the departed Zack Bronson at free safety. Mike Rumph moves in as the regular starter at right cornerback in place of the departed Jason Webster, though Rumph started 13 games there in place of an injured Webster last year. Second-round draft pick Spencer and sixth-rounder Lewis bring some fresh talent and potential at cornerback and safety, respectively.

What's to like: Rumph, big and physical, has displayed significant development on the edge and could be a major factor in coverage this year. He and Plummer give the Niners solid potential on the corners to match up with the big NFL receivers of today, though opponents continued to pick on Plummer during the preseason. Plummer, however, is the kind of player who shrugs off failures and mistakes and gets better as a game goes along. Parrish has played at an All-Pro level since joining the 49ers two years ago, and he enters his seventh NFL season in the prime of his career. He is a versatile ballhawk who likes to hit and can play both safety positions. Williams and Spencer provide an upgrade this season as the Nos. 3 and 4 corners, with both possessing the ability to play at a starter's level. Lewis is an aggressive, heavy hitter who is ahead of where everybody thought he would be at this point as an all-around safety. Carpenter's athleticism and potential still could play a factor this season at the safety position.

What's to wonder about: The Niners continued to struggle on third down in the preseason, just as they have done for years, and the onus for improvement falls on the cornerbacks, at whom most of those third-down passes have been directed. Plummer and Rumph both are on the verge of being very good cornerbacks, but inconsistency suggests neither is quite there yet, and Plummer can be beaten when targeted by opponents. Williams and Spencer are full of promise, but both still are unproven as the third cornerback, with Williams displaying inconsistency in the No. 3 role. Williams will play the first month of the season with a cast on his right hand to protect a broken thumb, which can't help. Parrish missed the final two preseason games with a torn calf muscle. Heard is a smart player and good tackler, but he lacks athleticism and quick instincts, and that could create problems at free safety. Carpenter has the athleticism and hitting ability, but he proved during the summer that he is not ready to handle the starting duties at free safety, which was a disappointment to the team, since Heard would be better as a backup. Carpenter doesn't have the necessary feel for the position, and he'll enter the season with a knee injury that will keep him out of the opener.

Key preseason stat: In their four exhibition games, the 49ers ranked 27th in the NFL in passing defense, 30th in stopping opponents on third down and 25th in average yards allowed per pass play.

The bottom line: This secondary still has the potential to be San Francisco's best in several seasons, but it didn't play that way on any kind of consistent basis during the preseason. There is still plenty of promise, however, once the front-line players on that unit begin playing together on a regular basis. The Niners appear solid at three starting positions and their nickel and dime coverage packages could be considerably improved. The big question is at free safety, where Heard must produce as the last line of San Francisco's defense. If he does, and the secondary gets help from a consistent pass rush, this could be a very productive unit.

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