He could have surgery as soon as Thursday, pending a potential second opinion, a source told ESPN.
Smith was hurt on a running play and went to the ground as he went to make a block on linebacker Leighton Vander Esch. He was able to walk to the locker room without any help, and the initial tests indicated the ACL in his left knee was intact.
Smith was spotted leaving the locker room after practice and did not appear to be limping. Left guard Connor McGovern spoke with him afterward.
"He gave me a nice reassuring grunt like he usually does," McGovern said. "That's the one thing playing next to him for so long, I can decipher his grunts. I think he's all right."
The loss of Smith, an eight-time Pro Bowler and protector of Dak Prescott's blind side, creates a gaping hole on an offensive line that was already dealing with questions before the Sept. 11 opener against the Tampa Bay Buccaneers.
Dallas drafted Tyler Smith in the first round to be the left tackle of the future and has used him exclusively at left guard during training camp and the first two preseason games. Smith is also dealing with an ankle injury that kept him out of the past two practices.
The Cowboys have played Josh Ball, who was selected in the fourth round last year but did not play as a rookie because of an ankle injury, at left tackle with the first team when Smith has taken breaks. They also drafted Matt Waletzko in the fifth round this year, but he only returned to practice this week after dealing with a shoulder injury for most of training camp.
Coach Mike McCarthy has singled out Aviante Collins' play during camp, but Collins has not played in a game since 2019. He spent most of last season on the Cowboys' practice squad.
The Cowboys could look for veteran help either in free agency or the trade market to replace Smith. Eric Fisher, the former No. 1 overall pick in 2013, is unsigned after starting 15 games last season for the Indianapolis Colts.
The Cowboys have more than $20 million in salary-cap space, according to NFL Players Association figures, but a good portion of that is scheduled to be directed to the practice squad and in-season call-ups.
Dallas is already breaking in a new right tackle with Terence Steele replacing La'el Collins, who was cut in the offseason. Tyler Smith and McGovern have been in a competition to be the left guard, and McCarthy said McGovern would be the starter if the season started today.
Tyler Smith took snaps during organized team activities and minicamp at left tackle. He started 12 games at left tackle last season at Tulsa and nine in his redshirt freshman season in 2020. The Cowboys like his strength and makeup but wanted him to focus on one position during training camp.
Necessity could change things with less than three weeks before the Buccaneers opener.
As for Tyron Smith, this continues a string of injuries in recent years. He has not played a full season since 2015 as he has dealt with knee, ankle, neck and elbow injuries. He missed 14 games in 2020 because of neck surgery. He missed six games last season due to an ankle injury and a stint on the COVID list.
Smith, who is the longest-tenured Cowboy, turns 32 in December. He is signed through 2023.