Manning was spotted with the unusual ball -- covered in neon green lycra -- Wednesday, but didn't want to say why he was carrying it around.
"That's Denver Broncos offensive business," he said at the time.
Fox, though, admitted Peyton and wide receiver Demaryius Thomas were carrying the balls in reaction to last season's trouble with fumbles.
"We're making an emphasis from Day One on ball security," Fox said. "We started with two of our better players in Demaryius and Peyton. What better guys to start with?"
The Broncos as a whole had a bit of a ball control issue last season with 27 fumbles, six of which Manning was responsible for.