The Chargers primary defense is the spread defense...I would say it's run at least 25% of the plays, significantly more on passing downs. Right now there are four players in that spread defense learning a new position within it: Cromartie, Weddle, Cason, and Gregory. Add in Tucker starting for the first time and Cooper missing due to a suspension, and it's somewhat easy to see why the defense looks poor to start the season. However, remember the Chargers rightfully didn't allow a point in the second half to Denver and that seven came on a short field following a blown call and 'malfunction'. Sproles also ran a kick back in the first half, which gave Denver the ball back spiking their yards total.
Once all the Chargers learn their new roles in the spread defense they'll once again have the best pass coverage, like the end of last season.