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.

Chiefs, 49ers trade

The Kansas City Chiefs have acquired cornerback Kenneth Acker from the San Francisco 49ers for an undisclosed draft pick.

A sixth-round selection by the 49ers in 2014, Acker missed his rookie season because of a foot injury. But he played in 15 games last season and had three interceptions.

The Niners made several other roster moves after the deal was announced on Saturday. The team released receivers Jerome Simpson and DiAndre Campbell; running back Kendall Gaskins; defensive lineman Darren Lake; kicker John Lunsford; linebacker Wynton McManis; offensive linemen Blake Muir and Ian Silberman; and cornerback Cleveland Wallace.

The team also placed receiver Eric Rogers on injured reserve.

Watson out for season

Ravens tight end Benjamin Watson has torn his right Achilles tendon and is likely lost for the season.

The injury occurred on Baltimore’s first offensive play in a preseason game against the Detroit Lions on Saturday night.

Watson appeared to trip while running a pattern and slumped to the turf. After he finally got up, he had to be helped off the field.

Signed as a free agent in March, Watson was expected to be a key element in the Ravens’ passing game. He had 74 catches for 825 yards and six touchdowns last season with New Orleans.

Austin gets extension

Wide receiver Tavon Austin has signed a four-year contract extension with the Los Angeles Rams.

The deal was announced while the team was in Denver for an exhibition game Saturday night. NFL.com reported the deal was worth $42 million with $30 million guaranteed.

“Tavon has been an integral part of our offense and special teams since we drafted him in 2013,” coach Jeff Fisher said in a statement.