Midseason report: Defense

First-half storyline: The 49ers have only four defensive players back at the same starting position from a year ago. And each of the changes has been for the better. New defensive coordinator Vic Fangio and his staff are utilizing the pieces perfectly to form what has been the strength of the team. Three-quarters of the secondary is comprised of new starters. Cornerback Carlos Rogers has been a significant upgrade in coverage over Nate Clements, whom the 49ers released after the lockout. Tarell Brown replaced Shawntae Spencer, who remains on the roster but has been demoted to the team's fifth cornerback. And Donte Whitner has played well in the safety position occupied last season by Reggie Smith and Taylor Mays, given away in a trade to Cincinnati. Meanwhile, the 49ers might have the best front seven in the NFL. General manager Trent Baalke made all the right moves along the way. The 49ers replaced Takeo Spikes with speedy youngster NaVorro Bowman at inside linebacker. And after showing little interest in retaining nose tackle Aubrayo Franklin, the 49ers have improved with Isaac Sopoaga at the nose and Ray McDonald at left defensive end

MVP: Middle linebacker Patrick Willis. Again, Willis is the team's best defensive player. And as the 49ers open the second half of the season with a 7-1 record, Willis must be considered on the short list of players in serious contention for NFL Defensive Player of the Year. Willis and Bowman have formed a sideline-to-sideline tackling duo unmatched in the NFL. Willis was outstanding in pass coverage, too. He also forced three fumbles and recovered two in the first eight games.Biggest surprise: Brown. He started five games in his first four seasons, and was never able to hold onto a starting job. This season, he has been solid while starting the first eight games at right cornerback. Brown registered seven passes broken up.Biggest disappointment: The 49ers experienced some injuries in the secondary that forced a bunch of different combinations early in the season. Spencer was penciled in as a starter, but he fell so far behind due to hamstring and toe injuries that, now, he does not even play. Dashon Goldson, Whitner and Tramaine Brock missed games due to injuries. Rookie Chris Culliver took advantage of the injuries to Spencer and Brock to win the job as the No. 3 corner.Best play: Justin Smith, a 285-pound defensive end who played every defensive snap in the game, preserved the 49ers' Week 4 come-from-behind victory over the Philadelphia Eagles with a play Fangio called, "The defensive equivalent of 'The Catch.'" Smith began the play as a pass-rusher and then hustled after speedster Jeremy Maclin. With two minutes remaining, all the Eagles needed was a field goal to take the lead. Smith caught up to Maclin and stripped him of the football at the 49ers' 31-yard line. The 49ers recovered, and they held on for a 24-23 victory, which served as a springboard for a 7-1 start and five-game lead in the NFC West.Worst play: The 49ers saw their 10-point fourth-quarter lead over the Dallas Cowboys evaporate and the teams went to overtime in Week 2 at Candlestick Park. Rogers and Whitner bit hard on Tony Romo's run fake, allowing seldom-used receiver Jesse Holley to get behind the secondary. The 77-yard pass set up the Cowboys' winning field goal, and dealt the 49ers their only loss through eight games.Key to the second half: The 49ers' front seven played a lot of snaps in the first half of the season, so they have to keep those guys from wearing down. The only non-starter who sees a lot of key snaps is rookie Aldon Smith, who leads the 49ers with 6.5 sacks. Smith must continue to generate good pressure on the quarterback in nickel situations to take heat off the 49ers' secondary.