FRISCO, Texas (AP) -- Tony Romo is out with yet another back injury and it's unknown when he will return, although Dallas coach Jason Garrett says he expects his star quarterback to play this season.
Garrett said Saturday that Romo sustained a broken bone in his back when he was hit from behind by Seattle's Cliff Avrill and slid awkwardly on the third play of a preseason game.
Romo tried to get back into Thursday's game and said afterward that he was OK. But Garrett said the 36-year-old woke up Friday with stiffness, and an MRI revealed Romo's fourth back injury in less than four years. The injury will not require surgery.
Garrett wouldn't rule out Romo for the regular-season opener Sept. 11 against the New York Giants. Rookie Dak Prescott, a fourth-round pick who has had a strong preseason, is the presumed starter, although Garrett wouldn't acknowledge that either.
"If you guys remember, he has played with fractures in his back before," Garrett said, referring to Romo's quick return from a small fracture in his back in 2014.
"So that probably more than anything else is what is not giving us a timetable. We've heard a wide range of possibilities in terms of when he would be able to play."
The Cowboys plunged from 12-4 in 2014 to 4-12 last season, when Romo missed 12 games with a twice-broken left collarbone. Dallas went 1-11 without him.
Romo had back surgery twice in 2013, the first time during the offseason and again in December after rupturing a disk in Week 16 against Washington and missing the finale that the Cowboys lost with a playoff berth on the line.
In 2014, the four-time Pro Bowl player sustained a small fracture in his back and missed a loss to Arizona before leading the Cowboys to the NFC East title and their first playoff win since 2009.
Garrett said Romo's injury won't end his season.
"It's not related to the other back issues that he's had," Garrett said. "There is a specific to the hit he took the other night in the game."
Center Travis Frederick said the team was surprised by the news on Romo when they reported to the team's new practice facility Saturday.
"Still trying to take it in a little bit," Frederick said. "The name of the game for us has always been focusing on taking things one day at a time and also have the `next man in' mentality. Being able to have Dak step up and be able to take over that role and just support him in any way that we can."
Prescott was elevated to the backup job when incumbent Kellen Moore broke his right ankle in a training camp practice.
The former Mississippi State standout has a preseason passer rating of 137.8, completing 39 of 50 passes for 454 yards with five touchdowns and no interceptions. He also has two rushing TDs.
The Cowboys are likely to add a veteran if Romo misses extended time, but now it figures to be as a backup to Prescott rather than the next in line behind Romo since Dallas elected not to bring in help soon after Moore's injury.
"Dak has done a nice job really at every turn starting back at training camp," Garrett said. "I thought he did a particularly good job in this game against Seattle because he certainly didn't anticipate going in on the fourth play of the game."
Now it looks as if Prescott will start in his regular-season debut, just as he did in the preseason in the first NFL game in Los Angeles in more than 20 years.
