On the Bears’ sideline Thursday against the Jacksonville Jaguars, guard Kyle Long tried to pry the ball from rookie running back Ka’Deem Carey’s hands. When Carey protested, Long explained a trainer would hold the ball for safekeeping, a memento of Carey’s first pro touchdown — albeit in the preseason. ‘‘He was like, ‘You gotta let go of it,’ ’’ said Carey, a two-time All-American at Arizona. Almost seven minutes later, Senorise Perry didn’t have time to consider where his football was stored after a five-yard touchdown run that gave the Bears their decisive 20-19 lead against the Jaguars. ‘‘We had to go out there for a two-point conversion,’’ said Perry, an undrafted free agent from Louisville. Two weeks into the preseason, rookie thrills are about only positive surrounding the Bears’ rushing offense. The team is searching for two players to back up Matt Forte and waiting for runners to separate themselves.