The Nationals are expected to purchase the contract of catcher Carlos Maldonado from Triple-A Syracuse, according to a baseball source. Maldonado is expected to join the club in San Francisco on Monday night. He will be the backup catcher, while Wil Nieves will get most of the starts during Rodriguez's absence.
Manager Jim Riggleman had originally planned to have Mike Morse as the backup catcher, but those plans were scrapped with arrival of Maldonado.
It was clear late Sunday afternoon that general manager Mike Rizzo had to make a move, because Rodriguez told some of his teammates that he wasn't going on the 10-day road trip, which starts Tuesday against the Giants.
Rodriguez, 38, was hitting .325 (40-for-123) with one home run and 16 RBIs in 35 games for the Nationals this season.
Defensively, Rodriguez has had a hand in cultivating a pitching staff that has reduced its ERA by 0.45 runs compared with last season.
As for Maldonado, he is considered an excellent catch-and-throw player. He played in 26 games for Syracuse and is 19-for-87  with a home run and nine RBIs. Maldonado, 31, is a 15-year veteran and had played most of his career in the Minor Leagues.
The last time he played in the big leagues was in 2007 with the Pirates, so he is familiar with relievers Matt Capps and Sean Burnett and outfielder Nyjer Morgan.
Maldonado has played 21 big league games during his career and has a .163 batting average with two home runs and four RBIs.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.