Why To Watch Thirteen down, three to go in the Colts' pursuit of a perfect regular season. Next up: a fickle Chargers team that has looked dominant in wins over the Giants and Patriots but soft in losses to the Eagles and, most recently, the Dolphins. Playing on the fast track inside the RCA Dome does not help the Chargers' chances, but there are a few good reasons to believe they are capable of pulling off the streak-snapping upset of the Colts -- RB LaDainian Tomlinson, TE Antonio Gates and an athletic corps of linebackers being chief among them. When the Chargers have the ball Rushing: Many things must go right for a team to knock off the undefeated Colts, but establishing a dedicated and effective rushing attack has to rank among the most important. The more effective the Chargers are running the football, the more time they will bleed off the clock, which in turn will limit the Colts' offensive opportunities. The Colts have great depth along their defensive line, but their defensive front seven lacks overall size, which makes it susceptible to a consistent north-south run game. The unit has been able to mask that weakness by generating turnovers and jumping out to early leads that allow this group to do what it does best -- rush the quarterback. The Chargers lack ideal athletic ability along their offensive line, but they have the size and strength to overwhelm the Colts' defensive front if the team is able to get Tomlinson 25-plus carries. The most important individual matchup in this facet of the game will be between ROG Mike Goff and LDT Corey Simon, who is clearly the Colts' best run-defending defensive lineman. Simon is still a bit overweight and tends to wear down quicker than he has in the past, but he has the initial quickness, size and strength to take up space in the middle, which has been a big help in terms of freeing up LBs Gary Brackett, David Thornton and Cato June to pursue the run. Goff is not a great pass protector and will struggle on later downs in that facet, but he does possess the size, strength, toughness and experience to handle Simon one-on-one as a run blocker. If that's the case, it will help free up OC Nick Hardwick and FB Lorenzo Neal to chase down the athletic but undersized trio of Colts linebackers. Passing: When the Chargers are clicking offensively, they can be one of the most difficult units to defend in the NFL because of their versatility and balance. On top of Tomlinson's potential to carry a heavy load on the ground, his versatility as a receiver also gives the Chargers an ability to keep the Colts' defense honest in this game. Much like the Colts with RB Edgerrin James and TE Dallas Clark, the Chargers can come out in a three-receiver or even a traditional two-back, one-tight, two-receiver personnel package but empty out the backfield and "flex" TE Antonio Gates out into the slot and put a lot of pressure on the opponents' back seven in terms of matching up in coverage. The Colts are fortunate to have athletic outside linebackers in Thornton and June, who are potentially capable of matching up one-on-one versus Gates and Tomlinson. However, these type of spread formations will put a lot of pressure on DSs Mike Doss and Bob Sanders in terms of deep support. If Doss and/or Sanders get caught up in all the traffic over the middle, WRs Keenan McCardell and Eric Parker are fully capable of making plays down the sideline versus the Colts cover-2 or cover-3 looks. While the skill-player individual matchups slightly favor the Chargers, the Colts own a massive advantage in terms of athleticism in the trenches. Starting DTs Simon and Montae Reagor usually give way on passing downs to LDE Raheem Brock (moves inside) and either Larry Tripplett or Josh Williams, which gives the Colts fresh legs to put pressure on QB Drew Brees. Even more concerning for the Chargers are the potential perimeter mismatches, as RDE Dwight Freeney (9.5 sacks) and pass-rush specialist Robert Mathis (11.5 sacks) should absolutely dominate their one-on-one matchups versus OTs Shane Olivea and Leander Jordan, who has been inserted in the starting lineup for the injured Roman Oben. Simply put, if the Chargers are put in a position where they are forced to abort the run on a consistent basis in this game, the Colts front-four pass rush will take over. When the Colts have the ball Rushing: The Colts continue to place a strong emphasis on the running game. James has averaged 25.7 carries so far this season and is coming off a 30-carry performance versus the Jaguars. By keeping James heavily involved on the ground, it helps to keep opposing defenses honest while also setting up the play-action passing series for Manning. There has to be some concern, however, regarding James' durability moving forward. In the first seven outings of the season, James averaged fewer than four yards per carry only once, but he has failed to reach that plateau in four of his last six games. With home-field advantage locked up, the Colts may begin to ease James' workload in order to give his legs some time to rejuvenate. If that's the case, backup RB Dominic Rhodes will see his carries increased. No matter how many carries James gets, establishing a solid rushing attack versus the Chargers' No. 1 ranked run defense will be no easy task. The Colts offensive line has been extremely efficient opening up holes on the ground this season. It is not a group with great size or overpowering strength, but the unit as a whole possesses outstanding quickness, toughness, technique and communication. The key for the Colts will be to generate a surge versus an imposing front-three of NT Jamal Williams and DEs Luis Castillo and Igor Olshansky. In all likelihood, OC Jeff Saturday will consistently need guard help versus Williams, which will cause a trickle-down effect for the rest of the line. If ILBs Randall Godfrey and Donnie Edwards continue to take full advantage of the protection, the Chargers will be able to adequately defend James and Rhodes on the ground. Passing: The Chargers have been outstanding versus the run, but their pass defense continues to get worse as the season progresses. For starters, the unit is not getting the same pass-rush pressure it was getting earlier this year. Rookie LOLB Shawne Merriman flashed great potential as a perimeter pass rusher during the middle of this season, but he has clearly hit a rookie wall and is no longer showing the same explosiveness off the edge. Castillo, Williams and Olshansky are doing an excellent job of plugging gaps and disrupting the run, but they have not provided much of an inside push in the passing game. As a result, the Chargers' secondary has been forced to hold up too long in coverage in recent games. If that's the case this week, Manning will absolutely torch the Chargers defense via the air. Edwards remains a very good cover linebacker and should help to keep James in check as a receiver in the backfield. However, that's where the favorable matchups end for the Chargers in this facet of Sunday's game. WR Marvin Harrison presents the biggest mismatch of all, as he will be working against inconsistent LDC Quentin Jammer, who has been beat deep entirely too frequently in the last several games. In order to protect against that mismatch, Chargers defensive coordinator Wade Phillips will be forced to roll his safeties to Jammer's side, which will open up a lot of room for WRs Reggie Wayne and Brandon Stokley to work high-low combination routes on the opposite side versus RDC Drayton Florence and nickel DC Sammy Davis. Furthermore, with so much attention expected to be given to the blitz package and three receivers, TE Dallas Clark could turn in one of the most productive games of his season as a seam-stretching pass catcher.