A back injury will sideline cornerback Richard Marshall for Sunday’s game at Cincinnati, leaving the Dolphins short-handed against a Bengals offense that ranks seventh in the league in passing yards and second in passing yards per play.

With Marshall out, Nolan Carroll is expected to make his fifth start in three-plus seasons, and safety/nickel back Jimmy Wilson could assume No. 3 cornerback duties, with cornerback R.J. Stanford also potentially in line for playing time.

“I’m excited,” Carroll said, adding that he is a better player this season because of improved technique and confidence.

Carroll has allowed 11 of the 23 passes thrown against him to be completed for a 14.3-yard average. Marshall’s numbers have been worse: 14 for 23 for a 14.8-yard average, according to Pro Football Focus.

Carroll “has played well,” Dolphins coach Joe Philbin said. “We would like him to make more plays on the ball at times, just like some of our other defensive backs. He’s intelligent, and you know what you’re going to get when you put him in the game. You’re going to get a great effort from the kid.”

Though Carroll was listed as probable on the injury report with an Achilles’ condition, he said he feels fine and sat out practice Wednesday only because coaches wanted to rest him. This Sunday, Dolphins cornerbacks must deal with Bengals receiver A.J. Green, who ranks second in the league in receiving yardage behind Miami’s Brian Hartline. Sean Smith figures to cover Green for much of the day, which often would leave Carroll on Armon Binns.

The decision was made for Marshall to rest his back this week with the hope that he will be ready for the Oct. 14 home game against St. Louis.

GAFFNEY STUDYING

Receiver Jabar Gaffney, who signed with the Dolphins on Tuesday, said he knows some but not all of Miami’s playbook after studying until midnight or so every night this week. “It’s a lot of plays to try to learn in three days,” he said.

Philbin was noncommittal when asked if Gaffney knows enough to be a factor Sunday. “Possibly,” Philbin said. “Every day has been a little bit better.”

Gaffney has a good chance of eventually settling in as the Dolphins’ No. 3 receiver. He was running with the Patriots’ first team before a quadriceps injury sidelined him and eventually led to his release.

• Hartline’s appendectomy in June was a lot more serious than he or the team previously disclosed. Hartline told NFL.com this week that his condition was worsened by gangrene and complications that led to a blockage of Hartline’s intestines. Hospitalized for 12 days in June, he said he lost 25 pounds and couldn’t eat for 12 days without vomiting.

Hartline said Friday a doctor treating him told him that some people with similar symptoms were hospitalized for a day or two, but that others died, and he was somewhere in the middle. But Hartline said he never felt that his life was at risk.

•  Ryan Tannehill was added to the Dolphins’ injury report with a thumb injury on his throwing hand, but he practiced Friday and is expected to play Sunday. Tannehill was one of eight players listed as probable, along with Kevin Burnett, Reggie Bush, Carroll, Tony McDaniel (back from knee surgery), Marlon Moore, Paul Soliai and Austin Spitler. Bengals cornerbacks Jason Allen and Dre Kirkpatrick were listed as doubtful.

• After Week 4, the Bengals led the league with 17 sacks, including five by Fort Lauderdale St. Thomas Aquinas graduate Geno Atkins.