Coach Wade Phillips said Wednesday that Ball would replace Ken Hamlin, who will miss two to four weeks with a high ankle sprain.
It will be the first career start for Ball, a seventh-round pick in 2007. A cornerback during his first two seasons, Ball has split time between cornerback and free safety this season, getting limited playing time. He has 17 tackles this season, 12 of which have come on special teams.
Phillips acknowledged that there is some concern about the 6-1, 188-pound Ball's ability to play the run because of his size.
"But he's so aggressive," Phillips said. "We've seen him on special teams. He's really aggressive. He tackles well. He's just not a big guy."
Free will step in for Marc Colombo at right tackle, offensive line coach Hudson Houck said.
The 6-foot-6, 313-pound Free, a fourth-round pick in 2007, had minimal regular-season experience before filling in after Colombo suffered potential season-ending ankle and leg injuries in the first quarter of last week's loss to the Green Bay Packers.
"The exciting part of it is you've got another guy ready to go," Houck said. "He steps in. He got his turn. And we don't miss a beat."
Colombo hasn't missed a game since he became a starter at the beginning of the 2006 season. He's a mauler with a nasty disposition who was voted by his teammates as the Cowboys' offensive MVP for the 2008 season.
"Marc Colombo has been a heck of a player for us -- not only a good player on the field, but a great leader," offensive coordinator Jason Garrett said. "Everybody on our team responds to him and how he approaches the game. We certainly will miss him, but we're excited about Doug Free.
"He's been with us for three years now and really taken the right approach. He's improved as a player. We're excited about what he did the other day in the game against Green Bay and we're excited to see him play this week."
Tim MacMahon covers the Cowboys for ESPNDallas.com. E-mail firstname.lastname@example.org.