The 49ers and defending Calvin Johnson

Chris Culliver's interception for the San Francisco 49ers against Tampa Bay announced his arrival as more than just a rookie fill-in for injured veteran Shawntae Spencer.

Culliver, a third-round pick from South Carolina, could serve as the third corner against the Detroit Lions in Week 6 strictly on the merits.

In fact, as coach Jim Harbaugh told reporters Friday, the team went with Culliver against the Buccaneers even though Spencer, who has been slowed by a toe injury, could have played.

Which one will play in the nickel defense against Calvin Johnson and the Lions?

"Tune in on Sunday and find out," Harbaugh said.

As Grant Cohn notes, Greg Cosell of NFL Films told KNBR-AM radio in San Francisco that he expected Culliver to play a leading role in defending Johnson, who leads the NFL in touchdown receptions with nine.

"The one player who really stood out to me and I think will be critical this week when they play Detroit, and that’s Chris Culliver," Cosell told KNBR. "He’s now their nickel corner. He plays on the outside. He’s over 6 feet. He’s 200 pounds. He runs well. They play a lot of man coverage concepts with a safety helping so that their corners can really play physically with their receivers coming off the line of scrimmage. So, I think you’ll see Culliver matched up against Megatron this weekend."

The 49ers liked Culliver coming out of college for several reasons:

  • Raw physical talent: Culliver is 6 feet tall and has run the 40-yard dash in the 4.4s.

  • Versatility: Culliver converted from safety to corner for his senior season, giving him a broader perspective than a cornerback might normally possess.

  • Potential: Being relatively new to corner meant, at least in theory, that Culliver still had considerable room to grow at the position.

Culliver made his interception against the Bucs playing left cornerback against three-receiver personnel on a third-and-9 play, with starting corner Carlos Rogers moving inside to defend the slot on the other side of the formation.

The 49ers figure to face quite a few similar personnel groupings Sunday.

Only three teams -- Buffalo, Seattle and Philadelphia -- have run more plays than the Lions with three-plus wide receivers on the field, according to ESPN Stats & Information. Detroit has also used two tight ends quite a bit, but with more than two receivers on the field so frequently, the 49ers' nickel defense will be key.

2010-11 49ers Pass Defense

So far this season, the 49ers have done a much better job limiting long pass plays.

The chart at right shows how the 49ers' opponents have fared this season versus last on passes traveling longer than 20 yards past the line of scrimmage. They are one of two teams yet to allow a touchdown pass on these throws. They've allowed only three completions on such passes after allowing 24 all last season.

Thanks to Hank Gargiulo and Doug Clawson of ESPN Stats & Information for providing the chart info.

2011 49ers Snap Counts