The 21-year-old had the ball stripped from his grasp in his side's pre-season win over the St Louis Rams and fumbled last week in their game against the Seattle Seahawks.
Fox told reporters: "I personally thought his progress was stopped; otherwise he's got to get on the ground faster or do a better job of holding onto the ball when guys yank on him late in the down.
"Again, every one of these things is a learning experience for these guys. I think he will work on it, so I have not lost confidence in him whatsoever."
Denver open the regular season against the Baltimore Ravens on September 5.