Romo is out for the season after breaking his left collarbone for the second time this season in the Thanksgiving Day loss to the Carolina Panthers. He will not need surgery, and the club believes time off will be the best medicine.
"We think Tony has a lot of football left in him," Garrett said.
Romo, who turns 36 in April, had two back surgeries in 2013, including a discectomy that has limited him from practicing full-time the last two years. Since 2010, Romo has broken his left collarbone three times.
"We do think [the collarbone] is an injury that will heal," Garrett said. "We do think the back injury has gotten better and better over time. He's more able to handle the work day-to-day and the work week-to-week, so I think that's improved. We do think the collarbone will heal. He's got a lot of football ahead of him."
The Cowboys have yet to move Romo to injured reserve, but Garrett said they anticipate moving up Kellen Moore from the practice squad to be Matt Cassel's backup.
Garrett said Romo should be good to go for the start of the offseason program. But first Romo will have to get over the disappointment of seeing his season end.
"Tony's a great competitor, Tony wants to play as much as anybody, wants to play at a high level and help our team, and when that gets taken away from you early on in the season, you have to watch, that's challenging. And then you get a chance to come back and then it happens again, those are not easy," Garrett said. "There's a tremendous investment that everybody makes in this, and the commitment that he's made to our team over the course of his career, the investment he has made has been significant -- when you can't go out there and play because of injury -- that can be frustrating for everybody.
"But somehow, some way, you've got to get your mind right, and you've got to start again. It's a clean slate, think about the right things to get yourself back with your rehab and your treatment and get focused on the future. And he's certainly doing that even though it's not an easy thing to do."