The Nationals re-signed Krook to a major-league contract prior to Sunday's game against the Athletics.
Krook had elected free agency after rejecting an outright assignment to Triple-A Rochester when Washington removed him from its 40-man roster a week ago, but he quickly ended up returning to the Nationals on a new deal. The 31-year-old will give Washington another left-handed option in its bullpen after PJ Poulin (forearm) was placed on the 15-day injured list Sunday.