Mayock isn't alone in that belief.
In his latest NFL Notebook, NFL Media's Albert Breer relayed quotes from a trio of defensive coaches dreading the prospect of Kaepernick as a dual-threat quarterback in Kelly's read-option offense.
"I think [Kaepernick] is a good enough passer, but obviously what'll be a nightmare is his ability to run," on defensive coordinator said. "That offense is straight 'Freddy Krueger' when you have a quarterback that can pull the ball and run at any given time."
Kelly's offense stalled out the last two years in Philadelphia because he didn't have a quarterback with the dynamic athleticism to pull off the second part of the zone read.
"The fact that Kap can make guys miss and get in the open field, they didn't have that last year at all in Philly," another defensive coordinator said. "We treated [Sam] Bradford like he was under center. There was zero threat of him running the ball. We told our guys, 'Don't treat him like he's in the shotgun, he's never gonna pull the ball.' "
Now that Kaepernick is poised for a return to the zone read attack, one rival defensive coach told Breer the combination with power back Carlos Hyde "could be scary."
Hyde starred in Urban Meyer's spread offense at Ohio State and stands to benefit from defensive fronts hedging against the threat of Kaepernick's legs.
Kelly has yet to anoint Kaepernick the starter for 2016. If he's still on the roster by April 1, Kap will count $15.9 million against the 49ers' salary cap.
Judging by the reaction of defensive coaches, that's a price worth paying.