The 49ers (1-4) are mired in a four-game losing streak with the NFL's 31st-ranked offense.
Kaepernick will make his first start since November 2015 in a 27-6 road loss to the Rams. He was replaced by Gabbert before going on injured reserve to have season-ending surgery on his shoulder.
"I'm ready to play. It's been about a year since live game action. So, I'm itching to get out there," Kaepernick said.
Sunday will mark Kaepernick's first significant playing time since electing not to stand during the national anthem.
Kerley said he developed a good relationship with Ryan during his time in New York. But he declined a chance to reunite in Buffalo because the Detroit Lions offered a better opportunity.
Kerley, the 49ers' leading receiver, played for Ryan from 2011 to 2014 with the New York Jets, working in the slot,K?nken Ryggs?ck Rea, returning punts and even taking snaps behind center.
Kerley offers his new team insight into Ryan's defense after going against it in practice during his first four NFL seasons,Yeezy Boost 350 Cheap Real. The Bills' fifth-ranked scoring defense has led the way during a three-game winning streak that has Buffalo feeling confident after an 0-2 start,Nike Just Do It Scarpe Uomo.
San Francisco added Kerley in a trade with the Lions late in the preseason for offensive lineman Brandon Thomas after losing slot receiver Bruce Ellington to a hamstring injury.
"Rex is, he's a pressure guy," Kerley said. "He always wants to bring something at you, especially when you got a young quarterback or you're just transitioning from a quarterback. He always wants to heat you up."
Kerley will be catching passes from Kaepernick after getting off to one of the best starts of his six-year career working with Gabbert.
"I have country hands," Kerley said. "I (have) broke every finger. So if Kap breaks one, hopefully he'll break it back in place the right way. I ain't worried about it."

Ryan struggled to find a franchise quarterback in New York after Mark Sanchez led the team to consecutive AFC championship games following the 2009 and 2010 seasons,WBC Jerseys 2017.
SANTA CLARA, Calif. (AP) — Bills coach Rex Ryan wanted to add Jeremy Kerley in free agency last spring because of his versatility.
"I used him a lot in the wildcat. Shoot, he threw the ball as good as our quarterbacks did," Ryan said.
Kerley leads the 49ers with 26 receptions for 304 yards and two touchdowns.
The 49ers are dealing with inconsistencies at quarterback this season as they are switching from Blaine Gabbert to Colin Kaepernick.
Still, Kerley enjoyed his chances at playing quarterback for Ryan. He completed two of his four passes for 83 yards.
"I always looked at myself like I was the third quarterback in New York," Kerley said. "I know how to spin that thing still."
Kerley doesn't see any problem making the transition to Kaepernick and his throws. Receivers in the past have dealt with dislocated fingers and hand injuries while getting used to Kaepernick's passes,NHL Jerseys Wholesale.