Head coach Gus Bradley said Friday that Rackley was undergoing further tests and might be a game-time decision.
"We are not going to push anything with Will Rackley, and will see how he feels leading up to the game and make our decision," Bradley said.
If Rackley can’t play, second-year player Mike Brewster would take his spot in the lineup.
Guy, a second-year player whom the Jaguars claimed off waivers from Seattle on Sept. 1, has played just four snaps on defense this season. Rookie Josh Evans has played every snap there since joining the lineup in Week 3 after Dwight Lowery suffered a concussion.
Evans has made 31 tackles, but has just one pass breakup and has struggled with bad angles and missed tackles.
"He [Guy] has shown more consistency, and we just felt like at the free safety spot we just needed to increase that level of play so we’re competing there," Bradley said. "I think it’ll re-direct his [Evans] attention to the competition part of it, and if that’s what he needs to help be focused, then we’ll do that."
Gratz, the Jaguars’ third-round draft pick, suffered a high ankle sprain in the season opener and missed the next five games. He has played in a reserve role the past two games, but will make his second career start on Sunday.