Flores, who was placed on the disabled list on May 9, was batting .311 with four homers and 18 RBIs in 26 games this season. He was 4-for-11 in three games rehabbing with Double-A Harrisburg. The team originally targeted Friday's game against the Philadelphia Phillies for Flores' return.
Instead, he will travel to Birmingham, Ala., on Monday to be examined by the renown specialist.
Also on Friday, the Nationals purchased the contract of right-handed reliever Mike MacDougal from Triple-A Syracuse, where he had two saves with a 3.24 ERA in 8 1/3 innings.
MacDougal pitched in five games with the Chicago White Sox this season, compiling a 12.46 ERA in 4 1/3 innings before being released. The former Kansas City Royals closer is 13-20 with a 4.18 ERA and 50 saves in his nine-year major league career.