Patterson was Washington's opening day starter in 2007. But he missed much of the last two seasons because of right forearm problems, limiting him to 15 starts in 2006 and 2007 combined. He had surgery in September to repair a nerve problem.
Still, he was considered in contention to get the start against the Atlanta Braves on March 30, when the Nationals begin this season at their new ballpark.
"We spent spring training evaluating him, and we came to the conclusion it was better to give our younger kids a chance to start," Nationals manager Manny Acta said.
"It's a rehabilitation-in-progress, and we can't afford to do that every five days."
In another move Thursday, the Nationals reassigned second baseman Bret Boone to minor league camp, saying he would begin the season at Triple-A Columbus. Boone, who turns 39 next month, came out of retirement to sign a minor league deal with the Nationals in February. He hasn't played in the majors since 2005.
The 30-year-old Patterson has never been able to recapture the form he showed for Washington in 2005, when he was 9-7 with a 3.13 ERA and 185 strikeouts in 198 1/3 innings.
In his final exhibition start for the Nationals, last Sunday against the Orioles, he allowed six runs and eight hits in four innings.
"What we did is best for both the team and John. It gives him enough time to get a job somewhere else," Acta said. "It would have been worse to keep him to the end."
Shawn Hill, another pitcher coming off surgery who at the start of spring training was thought to be in the running for the opening day start, will throw a couple of innings in a minor league game Monday.
"He's not going to be on the opening day roster," Acta said. "He's not going to have enough innings."