Latos, who had a chance to opt out of his Minor League deal on Monday, will likely pitch one more game for Triple-A Syracuse on Thursday against Pawtucket before joining the Major League club, the source said.
Once Latos joins the Nats, he will be reunited with manager Dusty Baker. The two worked together with the Reds in 2012 and '13. Latos won 28 combined games under Baker those two seasons.
Latos began the 2016 season with the White Sox, with whom he went 6-2 with a 4.62 ERA. Latos was released by Chicago on June 17, and he signed a Minor League deal with Washington 12 days later.
Latos has made six starts in the Nationals' Minor League system, posting a 1.64 ERA in three starts with the Rookie-level Gulf Coast League club and a 2.12 ERA in three starts with Syracuse.
Latos is 70-57 with a 3.57 ERA in 188 games (185 starts) with the Padres, Reds, Marlins, Dodgers, Angels and White Sox.
Bill Ladson has covered the Nationals/Expos for MLB.com since 2002 and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.