Fox refused to entertain reporters' postgame questions about making a switch at quarterback, The Denver Post reported.
Behind Orton, the Broncos kept faltering deep in Raiders territory. No possession was more painful than when Denver reached Oakland's 24, and Orton had tight end Daniel Fells open going into the end zone. The ball slipped out of Orton's hand, and defensive end Lamarr Houston pounced on it.
Fox stuck by his starter but told The Post after Monday's loss that Orton's game needed work.
"All and all, like all of us, it probably wasn't good enough," he said, adding that Orton's fumble was a "pivotal play, no doubt."
Orton completed 24 of 46 passes for 304 yards and one touchdown with one interception in an uneven effort.
The Associated Press contributed to this report.