By Simon Evans
Pouncey, who arrived in Dallas on Monday on crutches, has a high ankle sprain and has not been able to practice all week.
While his team mates went through a practice session at the Texas Christian University, Pouncey did rehab in the trainer's room. As the team left practice, Tomlin said: "He's out."
The talented rookie will be replaced by versatile Doug Legurksy for Sunday's clash with the Green Bay Packers.
Legursky, originally signed as an undrafted rookie by Pittsburgh in 2007, will play at center for the first time in his NFL career.
The Frankfurt-born 24-year-old has made just four starts for the Steelers at guard in his two seasons on the team.
Although Legursky will be up against highly-rated Packers nose tackle B.J. Raji, Tomlin was confident he would be able to cope.
"The NFL is made up of lots of players like him -- guys who somehow got an opportunity and seized it," said Tomlin.
"We're completely confident that he will seize this opportunity and play well. That's why we're not changing what we do."
Defensive end Aaron Smith, who has not played since October 24 after tearing his triceps, was also formally ruled out of Sunday's game.
Smith was never really in contention for a place and his left-end spot will again be taken by second-year defensive end Ziggy Hood.
(Editing by Mark Lamport-Stokes)