When the Nationals traded for Gio Gonzalez and signed Edwin Jackson their salting rotation became very crowded. The two guys who lost spots are John Lannan and Ross Detwiler, and Lannan might be on his way out via trade before Spring Training is over. According to Danny Knobler of CBS Sports, the Red Sox and the Tigers are the team that could most use Lannan's services.
The Red Sox and Tigers are believed to be the teams most focused on John Lannan, with a growing expectation that the Nationals left-hander will be dealt by the end of the spring.
Lannan was a 10-game winner and had a 3.70 ERA in 33 starts for Washington last season, but the Nationals rotation became much more crowded when the team first traded for Gio Gonzalez and then signed Edwin Jackson as a free agent. With Stephen Strasburg, Jordan Zimmermann and Chien-Ming Wang also expected to have spots in the opening day rotation, there's no room for Lannan or Ross Detwiler.
He goes on to point out that the Tigers haven't made an specific overtures to the Nationals about Lannan, but they have been interested in all available starting pitching this Spring, so it makes sense that they would check in on Lannan.