His 58-yard run around the left end set up the game’s first touchdown, a 5-yard strike from Cutler to Marshall.
Rookies Kyle Long and Jordan Mills held their ground at right guard and tackle.
Devin Hester also showed a spark, returning the game’s opening kickoff 45 yards. And reserve Michael Ford ran one back 100 yards in the third quarter, leading to a 3-yard scoring run by Michael Bush.
The first-team defense sacked Rivers three times and forced two turnovers, even though star defensive end Julius Peppers (hamstring), defensive tackle Henry Melton (concussion) and linebacker D.J. Williams (strained right calf) sat out with injuries.
The Chargers lost again after getting pounded by Seattle last week.
Rivers downplayed the turnovers, even though he got swarmed as soon as he stepped on the field. He was 5 of 9 for 50 yards with an interception, and he lost a fumble on a sack by Shea McClellin, leading to a 3-yard run by Forte that made it 14-0 near the end of the first quarter.
“I think that can be overanalyzed,” Rivers said. “They are what they are, but I don’t see it as a step back. I have seen some bad play and non-rhythmic offenses in the preseason, then they have great years, and I have seen some great drives in the preseason and they all go for nothing in the regular season.”
Backup Charlie Whitehurst came in early in the second quarter and was 6 for 9 for 69 yards and a touchdown. He fumbled away his first snap, leading to a field goal by Robbie Gould, but he also led the Chargers on an 80-yard TD drive late in the half.
Cutler got sacked twice on the opening drive, an ominous start after he threw an interception on Chicago’s first play from scrimmage the previous week.