Double-A Frisco shortstop Jurickson Profar singled in the fifth inning of Wednesday's game, extending his hitting streak to 26 games. He has reached base in 35 straight games.
Washington said Feldman, who made a spot start on Monday for Derek Holland, will likely not be available until Friday's game against the Astros.
Wednesday's game against the Athletics marked the 12th sellout at the Rangers Ballpark in Arlington, tied for the third most in team history. Thursday's game was also projected to be a sellout.
At 24-14, the Rangers are off to the second-best 38-game start in team history.