The Chargers have one of the most underrated defense in the league. Bosa, Ingram, Brown, Perryman (expected to return week 5-6), Verrett, Addae and Hayward form a fresh, young and playmaking defense that counts too with good players like Mebane, Luiget, Emanuel or Lowery. This is the first year of Lynn's Staff. Gus Bradley is a good cordinator despite establishing a new scheme. The guys have played together last year so there will be chemistry. The most involved players on the change will be Ingram and Perryman, but they are polivalent players so they will adapt to the 4-3 fast. Bosa played 4-3 in OSU. For Mebane and Liuget, the transition from 3-4 NT to a 4-3 DT should be easy. They screwed against the run so this new scheme should help while still having elite pass rush and a good secondary.
The offense have (IMO) a Top 5 RB, Top 5 receiving corps (Allen, M.Williams (returns week 4-5), T.Williams, Henry, Gates, Inman, Benjamin) and Top 10 QB. This team would be SB contender with a Top 10-15 OL, that fact remains to be seen. Injuries have been the issue of this unit last years(like the RBs and WRs.....). If healthy, Barksdale and Okung are good tackles, Slauson (an elite C) has been moved to LG, Center spot will be filled by Spencer Pulley or polivalent rookie Dan Feeney. With Forrest Lamp out for the season, Kenny Wiggins (the weakest piece of the unit) or even Feeney will play as RG. IF HEALTHY this is a solid line to run the ball well and protect Rivers.
IF HEALTHY this is a powerful aerial offense with a real threat in the ground game with Melvin Gordon. Keenan Allen is the less lucky guy in the world and he is an elite receiver that has chemistry with Rivers.
New staff, new defensive scheme, new playbook. That's why this line is so low. Remember that they count with the surprise factor and won 5 games last season without Allen, Verrett, Woodhead, almost the whole starting OL, etc... Most of their losses were close including some 4th quarter comebacks allowed by Mike McCoy.
I think they will have a high chance to assault the AFC West (Denver has not offense, Oakland has a weak defense and nobody knows what will do Lynch and Kansas have an old and punished defense while not having a good RB1 to make Smith's work possible )