An MRI on Friday revealed Romo's back injury is much worse than anticipated, Garrett announced Saturday, two days after he was knocked out of the game against the Seattle Seahawks.
"We're confident that he's going to be coming back and playing football for us this year," Garrett said but provided no timetable on Romo's possible return.
A source told NFL Media's Ian Rapoport that the best-case scenario for Romo's return is during the middle of the season. According to a Pro Football Talk source with knowledge of the situation, Romo is expected to miss six to 10 weeks.
Romo underwent a pair of back surgeries in 2013 and he missed most of last season with a twice-broken left collarbone. Garrett said the injury is not related to the surgeries Romo had in 2013.
The Cowboys went 4-12 last season, including 1-11 with three backup quarterbacks, as Romo missed 12 games.
On Dallas' third offensive play on Thursday night, Romo scrambled from the pocket, then went into a slide near the line of scrimmage. He was crunched by Seahawks defensive end Cliff Avril from the blind side.
After staying on the turf initially, Romo walked off on his own. He threw some passes on the sideline but was held out for the rest of the game by Garrett after lobbying to return. Romo completed his lone pass of the game for 11 yards.
"I was going into a slide," Romo said after the 27-17 loss to the Seahawks. "But in a weird way, I feel good about the fact that was probably as tough of a hit I've taken on the back as I've had in the last five years.
"From that regard, I feel very lucky that it can hold up and I can keep going."
Rookie fourth-round pick Dak Prescott likely will start the regular-season opener against the New York Giants on Sept. 11 if Romo is ruled out.