As the World Series champion Nationals await resolution on free-agent third baseman Josh Donaldson, they are putting together a possible Plan B.



The Nats have reached agreement with free-agent infielder Starlin Castro on a two-year, $12 million contract to play second base, according to sources and published reports. The deal is pending a physical, and is not the team’s only hedge against the possible loss of Donaldson to the Braves, Twins or another club.



According to sources, the Nats also have exchanged proposals with free agent Asdrúbal Cabrera, who was the Rangers’ primary third baseman last season before getting released on Aug. 3 and signing with the Nationals four days later.



Cabrera mostly played second for the Nationals, but also could slide back to third, perhaps in combination with Howie Kendrick, whom the Nats re-signed to a one-year, $6.25 million contract in December.



In addition to second and third, the Nats also...