Chargers defensive coordinator Ron Rivera is concerned about the offense Mike Martz might be building in Chicago. San Diego and Chicago will play a preseason game on Saturday. "We went back and we watched Coach Martz's offense and things he had done in the past," Rivera said Friday on "The Waddle & Silvy Show" on ESPN 1000. "We looked at some [film] later on San Francisco stuff where they started to get explosive down the end of the 2008 season. "Really saw some things that were, I don't want to say scary, but it causes a little bit of concern because of how they attacked vertically." Rivera, who was the Bears defensive coordinator from 2004 to 2006, understands the danger of attacking the wide-open system. "If you're playing coverage, you got to be over the top of everybody because if the quarterback hits the guy running down the seam, it's going to be a big play," he said. "If you bring pressure, you got to make sure somebody is running with everybody else, or it's a big play. There is some concern because its an explosive offense."