The Nationals said on their Web site that Guillen would have the elbow examined by noted orthopedic surgeon Dr. James Andrews on Monday in Birmingham, Ala.
Guillen, 30, was one of several veterans constantly mentioned in trade rumors, but the injury likely negates that possibility.
"Obviously, this means he won't be traded," Nationals general manager Jim Bowden told the Washington Post. "Our concern is for the player. That's our only concern, his health, his medical condition. I've called all the teams that had interest in Jose Guillen and told them that I can't trade them an injured player."
Guillen, who has struggled this season, was hitting .216 with nine home runs and 40 RBIs in 241 at-bats. He was placed on the DL late Thursday.