Soriano has said he would play nowhere but second base, where he achieved all-star status, and refused to take the field Monday for a game when Manager Frank Robinson put Soriano's name in the left field spot on the lineup card.
Washington's management informed Soriano and his agent that Soriano could either play left field or lose his $10 million, one-year contract.
Soriano played the position during a 9-3 win over St. Louis Wednesday. He had only one fielding chance and wound up picking a runner off second base after making a catch.
"I'm not comfortable," Soriano said after the game. "Today was easy because I only had one fly ball. Tomorrow might be different."