The Nationals designated Krook for assignment Sunday.
Krook will be pushed off the 40-man roster after the Nationals recalled first baseman/designated hitter Abimelec Ortiz for Triple-A Rochester in a corresponding move. The lefty reliever has struggled over eight appearances in the big leagues this season between the Athletics and Nationals, pitching to a 12.79 ERA, 2.37 WHIP and 7:6 K:BB in 6.1 innings.