Manager Ron Washington made the announcement Saturday morning just before the Rangers' bus left for Maryvale, where Texas plays the Milwaukee Brewers in Cactus League action in the afternoon.
"He deserves it," Washington said of Feldman. "He won 17 games [in 2009] and he's been throwing the ball well here this spring."
It had been assumed that Harden, signed to a one-year, $6.5 million contract during the offseason, would get the Opening Day nod, but the Rangers opted to go with the low-key Feldman instead.
Feldman will make his final exhibition start Sunday, Washington said, then have a live batting practice session three days later "to get his pitch count where we want it."
The Rangers also announced that they had optioned pitchers Brandon McCarthy and Derek Holland and outfielder Craig Gentry to Oklahoma City, pitcher Alexi Ogando to Frisco and assigned catcher Kevin Richardson to minor league camp.
Wilson's performance this spring has been an eye-opener for the Rangers, who love what he adds to the bullpen but saw him blossom as a starter, both in pitching and in attitude.
"If he was just going to be a back-end of the rotation starter, we would have probably kept him in the bullpen," Washington said. "He's shown a great work ethic and leadership.
"When you know you have a commodity like that in the bullpen and now you have to make an adjustment, you know it takes something special, and he was special."
McCarthy and Holland will join injured Tommy Hunter to form a pool of three starters at Oklahoma City that the Rangers hope to turn to when, not if, they need help in the rotation down the road.
Jim Reeves, a former columnist with the Fort Worth Star-Telegram, is a regular contributor to ESPNDallas.com.