The National Football League suspended Texans left tackle Duane Brown for four games Tuesday for violating the league’s drug policy, a punishment that now has sidelined two of the team’s starters.

Brown admitted he “unknowingly” took a substance considered a performance-enhancing drug. He joins outside linebacker Brian Cushing on the suspended list.

Even though 10 players are tested from each team every week during the season, it’s rare that two teammates are suspended at the same time.

Brown’s absence could not have come at a more difficult time for the Texans, who are 2-0.

His primary responsibility in this Sunday’s game against the Dallas Cowboys would have been outside linebacker DeMarcus Ware, one of the NFL’s premier pass rushers.

That assignment now goes to Rashad Butler, a five-year veteran who will make his first career start.

Besides missing the Dallas game, Brown, the Texans’ first-round pick in 2008, won’t play against Oakland, the New York Giants and Kansas City.

Nick de la Torre/Chronicle

Cushing, who tested positive for hCG — banned because it can be used as a masking agent for steroids — has two games remaining in his four-game suspension. He’ll return for the Giants game.

Brown didn’t disclose what supplement he took but apologized for his action.

“I take full responsibility for putting myself in this situation,” he said in a statement.

“I unknowingly took a supplement tainted with a banned substance and now have to deal with the consequences. After reviewing the appeal process and speaking with legal counsel, I have decided not to appeal my suspension. I understand the rules and accept my punishment.

“I vow to learn from this mistake and be a better player and teammate because of it.”

‘A significant blow’

The Texans can’t comment on the drug-testing policy. The NFL doesn’t comment on anything involving any player who is suspended.

“We are disappointed that Duane Brown has been suspended,” Texans general manager Rick Smith said in a statement. “This is a significant blow to our team.

“Our players have to understand that they are responsible for what goes into their bodies and that they take risks any time they use a supplement not approved by the NFL.”

In nine seasons, the Texans have had three players suspended. The first was deep snapper Bryan Pittman, who was suspended in 2008 for taking what he called a weight-loss drug.

“Our head athletic trainer (Geoff Kaplan), head strength and conditioning coach (Cedric Smith), as well as our team nutritionist (Roberta Anding) have had many meetings and presentations constantly reminding our players about the dangers of the supplement industry and risks associated with taking non-approved products,” Rick Smith said. “I am confident in our coaches and our players that we will work through this adversity and continue to play winning football.”

Butler welcomes chance

When the Texans return to practice Wednesday, Butler will line up at left tackle. Ephraim Salaam was re-signed to become the new swing tackle behind Butler and Eric Winston.

“It’s unfortunate what happened to Duane, but this is the kind of opportunity you hope for throughout your career but everybody doesn’t get,” Butler said. “You may only get it once, and you have to take advantage of it. I believe I will.”

Butler, 27, has been with the Texans for the last four seasons but has seldom played.

“It hasn’t been that tough being the swing tackle because I prepare like I’m the starter,” he said. “You know that opportunity could come at any moment. Somebody could go down with an injury. Something could happen like happened to Duane.”

Butler knows how good Ware is.

“Playing the Cowboys is a big game for us,” he said. “As the backup, you have to be excited about playing no matter who you’re going against. It’s a big game. He’s a great player. I’m excited about this opportunity.”

john.mcclain@chron.com