Bigby will enter training camp penciled in atop the depth chart. He is a seven-year veteran who has been productive when given ample playing time, intercepting five passes in 2007 and four in 2009. He comes over from Seattle, where he spent most of last season playing special teams and providing spot-duty on defense.
Taylor, who the Chargers traded up to acquire in the third round, seems destined to win the starting role at some juncture. Whether he can overtake Bigby as a rookie remains to be seen. He has the experience and intangibles to contribute early, but it is tough to ask a rookie to start in the secondary without a bull's-eye on his back.
Vaughn Martin vs Luis Castillo
Castillo missed the 2011 season with a broken leg and Martin took full advantage, starting the first 15 games of his career and posting 47 total tackles (second on the defensive line behind Antonio Garay). Martin still needs to show more of a mean streak and better utilize the elite athleticism that prompted the Bolts to take a flier on him in the 2009 draft, but he has shown marked improvement every year.
Castillo, who was released and re-signed earlier in the offseason, is healthy again and ready to compete for his old post as the starting left defensive end. Although Castillo hasn't topped 2.5 sacks since 2006 and has not exceeded 40 tackles since his rookie campaign, he anchors well against the run and keeps his linebackers clean. He does the dirty work asked of a 3-4 front man and makes the rest of the defensive front seven better.
Antoine Cason vs. Marcus Gilchrist
CB Antoine Cason
Gilchrist will benefit from a full set of OTAs and a longer training camp. He is a versatile defender who will challenge receivers and limit yards after the catch. If he can overtake Cason, the starting job may be his for years to come. If not, San Diego may decide to re-sign Cason, potentially tying Gilchrist to the bench for the foreseeable future.
Tyronne Green vs. Rex Hadnot
OG Tyronne Green
Nonetheless, the Chargers have made it clear the starting job will not be handed to Green. Early in the offseason, the team moved Brandyn Dombrowski back inside to guard. Smith fueled the competition even more by adding Hadnot, who started every game last season for the Arizona Cardinals. Green has more upside and ability than Hadnot, but the eight-year veteran will offer a strong challenge throughout training camp and possibly beyond.
Colin Baxter vs. David Molk
This battle is more important than it appears at first glance, as starter Nick Hardwick has missed 20 games due to injury over the last five seasons. Baxter spent most of his rookie year on San Diego's practice squad -- he also had a three-week stint with the New York Jets -- after the Chargers offered him a contract far exceeding that of their other practice-squad players. He plays with a nasty demeanor and can handle making line calls and adjustments.
Though Baxter is seen as a long-term project, those plans may change if Molk can win the No. 2 job. San Diego selected Molk in the seventh round, deeming him the highest-rated player left on its board. Molk has exceptional upper-body strength and uses his hands well. He is also superb at delivering shotgun snaps, which is vital in San Diego's offense.
Curtis Brinkley vs. Edwin Baker
At some point, the Chargers will add a veteran running back. Until then, Brinkley and Baker will challenge for the No. 2 job and the right to share Ryan Mathews' leftovers with Le'Ron McClain. Brinkley, entering his third season, showed well in a limited role in 2011. He shined in a Week 8 game at Kansas City, carrying 10 times for 43 yards and a TD and catching three passes for 24 yards; he also converted a two-point conversion to tie the game late in the fourth quarter.
Baker has an impressive resume in his own right, headlined by a dominant 2010 season at Michigan State in which he ran 207 times for 1,201 yards and 13 scores. His numbers dipped as a junior in 2011 as he fell from his perch atop the depth chart, but he remains a strong inside runner who runs with a low center of gravity. He is also extremely durable, which is key for any player who may be handcuffed to Mathews.
Dante Rosario vs. Kory Sperry
Norv Turner has acknowledged there is a good chance the Chargers will carry four tight ends. The frontrunner for that No. 4 spot is Rosario, who joined San Diego as a free agent this offseason. Rosario is an excellent receiving tight end who makes tough catches and can secure the ball in traffic. His best work came in 2009-2010, when he caught 58 passes as part of Carolina's conservative passing attack.
The incumbent Sperry is a better two-way player who has yet to fulfill his potential in the passing game (although he has made a few big receptions against defenses that slept on him). He is a diligent blocker and strong contributor on special teams. He first joined the Chargers as an undrafted free agent in 2009, so his familiarity with the team and Turner's offensive system will work in his favor.
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Michael Lombardo is a long-time contributor to the Scout.com network. His analysis has been published by the NFL Network, Fox Sports and MySpace Sports. He has followed the Chargers for more than 16 years and covered the team since 2003. You can see more of his updates by following him on twitter.