JACKSONVILLE – A shaky situation got shakier Thursday.

Because of that, the Jaguars' secondary could be more inexperienced Sunday than first thought.

With veteran Alan Ball already unlikely to play against the Miami Dolphins Sunday, cornerback Dwayne Gratz was limited in practice Thursday after experiencing tightness in a hip flexor during the two-hour session at the Florida Blue Health and Wellness Practice Fields.

"He just felt some tightness," Bradley said of Gratz Thursday as the Jaguars (1-6) prepared to play the Miami Dolphins (3-3) at EverBank Field in Jacksonville Sunday at 1 p.m.

"It was one of those where he felt a little bit, so we just pulled him out."

With Ball doubtful, Gratz and Demetrius McCray ideally would start at corner Sunday with McCray moving to Ball's right corner spot and Gratz returning to left corner, where he started the first four games of the season.

If Gratz can't play Sunday, Bradley said second-year veteran Jeremy Harris would start at right corner opposite McCray, with veteran Will Blackmon – the team's nickel corner – also figuring into the mix at cornerback if Harris or McCray were injured. In that scenario, safety Sherrod Martin would play the nickel.

It would be Harris' first NFL start.

"He's always been a guy who I felt really played tight coverage," Bradley said. "His thing that we've worked on is his physicalness. At times, those guys have to tackle on the perimeter. That's one area he needs to work out."

Ball on Thursday missed a second consecutive practice with a biceps injury sustained in a victory over Cleveland Sunday. He finished the game with the issue, but went to San Francisco Wednesday to receive a second opinion. Ball said Thursday he doesn't expect to play Sunday, but said he doesn't believe the injury is season-ending.

Bradley on Thursday said for a second consecutive day he considers Ball "highly unlikely" for Sunday.

Also around the Jaguars Thursday:

*Bradley said running back Denard Robinson on Sunday likely will play a similar featured role to last week, when he started and rushed for 127 yards on 22 carries. Bradley said the Jaguars aren't as concerned as much who starts at running back, with Toby Gerhart, Robinson, rookie Storm Johnson and Jordan Todman available depending on packages. …

*Bradley said he is looking forward to seeing how Gerhart plays after being out two weeks to rest a foot/ankle injury that hampered him in Weeks 1-5. Gerhart started those games, rushing for 123 yards on 48 carries. "You'll see Toby get quite a few reps," Bradley said. "I was really watching him today. He looks really good. I don't know if it's his legs are fresh, or if he finally feels healthy. But seeing him carry the ball and run, he looks like he's at another speed. I think we'll be able to judge him by how he does in the game." …

*Rookie cornerback Aaron Colvin did not practice one day after practicing limited. Wednesday was his first practice since sustaining a torn anterior cruciate at the Senior Bowl in January. Colvin spent the first seven games of the season on the Physically Unable to Perform list, with the team now having a three-week window from Wednesday to decide whether to activate Colvin or place him on injured reserve. "We want to try to bring him back gradually," Bradley said. …

*Defensive coordinator Bob Babich spoke to the media, addressing this week's loss of middle linebacker Paul Posluszny: "The guy was unbelievable for us. It's just a tough deal and everybody feels for him. Within the team we've got to get somebody to step on up. Our guys have done a good job of practicing with the linebacker unit. They really have stepped it up and the defensive players have rallied around that unit also. It's pretty cool to see." …

*Posluszny will miss the rest of the season with a torn pectoral muscle after undergoing surgery Wednesday. J.T. Thomas, a starter at Otto linebacker the past four games, will move to the middle with Dekoda Watson expected to start at Otto. "He's played all three positions at one time in our system," Babich said of Thomas. "He understands how the system works. Since he's been here he's gained a lot of confidence with his play. It's been cool to see." Babich coached Thomas during Thomas' first two NFL seasons in Chicago. "He's matured and he's become a pro and is doing a great job." …

*Babich on second-year defensive end Ryan Davis, who has excelled as an interior pass rusher in nickel situations this season and who will move into more of an every-down role with end Andre Branch out about six weeks: "He's shown that he can do a lot of different things. His pass rush has been really good but we like him playing the Leo position now on first and second down too. He's done that really well."…