Colombo had arthroscopic surgery on Aug. 16 to remove five loose particles from his right knee. On that day, Cowboys owner/general manager Jerry Jones said he expected Colombo to return within 10 to 14 days.
The Cowboys hope Colombo will be able to practice next week and play in the Sept. 19 home opener against the Chicago Bears.
Left guard Kyle Kosier is also expected to miss at least the season opener with a sprained MCL in his right knee suffered Aug. 18. That injury requires a recovery period of four to six weeks.
The 6-foot-2, 326-pound Holland, who the Cowboys re-signed to a two-year contract in May despite not activating him for any games last season, has 48 starts in his eight-year career with the Saints, Broncos and Cowboys.
The Cowboys acquired the 6-8, 316-pound Barron from the St. Louis Rams, trading recently cut linebacker Bobby Carpenter in an exchange of disappointing first-round picks. Barron started 74 games -- 42 at right tackle -- in five seasons with the Rams, who replaced him by drafting Rodger Saffold with the 33rd overall pick.
"It gives me a lot of confidence in those two guys," coach Wade Phillips said of the reserve linemen's experience. "Holland and Barron both have started and played and played well in a lot of games. That gives us confidence. And they've worked well in our system and worked hard. I don't think it's going to be perfect for them, but we have confidence that they'll do a good job for us."
Added Barron: "My confidence is high. This is not my first rodeo."
Colombo overcame a career-threatening knee injury suffered during his rookie season in 2002 with the Bears to become a reliable starter for the Cowboys. He started 57 consecutive games before suffering a broken right fibula and torn ankle ligaments that required surgery last season. He was sidelined for the rest of the regular season.
Kosier missed 13 games in 2008 due to a broken right foot, a key factor in the Cowboys failing to make the playoffs that season.
"This is the ultimate team game," quarterback Tony Romo said. "I think you have to have people able to step up and be in position to help you. We have a lot of confidence in the guys that are stepping up. They're good players as well. We're lucky enough to have some depth at those positions and we're going to keep battling."