Garrett said doctors advised Holloman to retire because of the injury. He missed seven games last season because of a spinal contusion.
Holloman was injured in the fourth quarter of the team’s second preseason game against the Baltimore Ravens after trying to make a tackle.
The second-year pro started the last two games of last season at middle linebacker, making 11 tackles and recording two sacks in the season finale against the Philadelphia Eagles.
Holloman, 23, was drafted by Dallas in the sixth round of the 2013 NFL Draft out of South Carolina.
- Scooby Axson