The Dallas Cowboys are uncertain whether Tyron Smith will be able to play again this season and are considering placing the star left tackle on injured reserve because of a neck injury, a team source confirmed to ESPN's Ed Werder.

Smith's neck injury flared up in practice following the Cowboys' season opener, and he missed the next two games before returning for Sunday's loss to the Cleveland Browns. But he has suffered a setback, the source confirmed, and missed practice Wednesday.

A source also confirmed that Smith is seeking a second opinion to find the best way to resolve the issue.

"He's further back than he was this time last week -- would be the best way to describe it," Cowboys coach Mike McCarthy said before Thursday's practice, adding that Smith would work with the rehab group.

NFL Network first reported Thursday the possibility that Smith could miss the rest of the season.

Smith has missed games in each of the past five seasons, dealing with back, hamstring, knee and elbow injuries in addition to the neck injury.

Brandon Knight started in Weeks 2 and 3 for Smith against the Atlanta Falcons and Seattle Seahawks, but he took over last week at right tackle after undrafted rookie Terence Steele struggled. Knight could move back to left tackle, with Cameron Erving, who is eligible to return off injured reserve this week, and Steele battling for the right tackle spot.

Erving suffered a knee injury in the season opener. The Cowboys added veteran Jordan Mills to the practice squad late last week. He has 84 career starts at right tackle.

"Just like any of the personnel evaluations, his experience is a benefit," McCarthy said. "He has come in, kind of went through a transition last week. I thought he did some good things in practice yesterday. So definitely, experience is important. You never have enough experience, especially at the tackle position."

The Cowboys have not had right tackle La'el Collins all season, and he had season-ending hip surgery on Wednesday.

The loss of both tackles would impact Dak Prescott most, but the quarterback has confidence the offense can succeed.

"Obviously, you can't replace some of the guys we have on this team and some of our original starters, but I have a lot of confidence that the coaches and the people in the front office make the right decisions putting these people here," Prescott said. "Not only that, I see the way these young guys come in and practice and prepare. For me it's about instilling confidence and allowing them to know that I trust in them, I believe in them and I'll go to war with them."

ESPN's Todd Archer contributed to this report.