SALT LAKE CITY — Utah starting running back Joe Williams has retired from football after a slow start to the 2016 season.

Coach Kyle Whittingham said the senior has some physical issues that forced him to leave the game. Williams was expected to replace current Denver Broncos running back Devontae Booker, but struggled and was benched during the last game after losing a fumble. Ball security had been an issue for Williams since 2015.

“His body is just worn out,” Whittingham told reporters Tuesday. “It has nothing to do with playing time or any bad feelings, he’s leaving on good terms. I think you noticed in some games that he’s just not the same guy. He has some things that are really bothering him physically so he’s going to retire.”

Williams rushed for 75 yards on 22 carries in two games this season and is on track to graduate in December. Freshman Zack Moss has been promoted to starter after rushing for 58 yards on 12 carries in his debut last week.

Whittingham also announced that defensive end Kylie Fitts will miss the rest of the season with a foot injury. Fitts was in his second season as a starter and led the Utes with seven sacks in 2015. The senior will apply for a medical redshirt.

Seniors Pita Taumoepenu and Hunter Dimick are listed as starters at defensive end.

Senior receiver Cory Butler-Byrd has been reinstated from an indefinite suspension. He was suspended amid a misdemeanor criminal mischief investigation stemming from allegations that he damaged a university police patrol car and building in July. Butler-Byrd has pleaded guilty in the case.

“He served his two-game suspension so we feel like he’s done everything that has been asked of him,” Whittingham said. “He still has things that he’ll adhere to the rest of the way, but he’s good to go.”

Butler-Byrd played cornerback in 2015, but made the switch to receiver to help an offense that has struggled through the air since joining the Pac-12.