The ligament reconstruction -- known as Tommy John surgery -- was performed by Dr. James Andrews, who examined Guillen in Birmingham, Ala., on Monday. An MRI exam revealed the full tear.
"He could not go forward without having this operation," Nationals general manager Jim Bowden said.
Bowden wasn't sure how long the injury would sideline Guillen, whose contract expires at the end of this season.
"It can be as early as eight months, it can be as late as a year and a half," Bowden said. "Everybody's different."
Bowden said he believed there was probably a particular throw that tore the ligament in half; Guillen left a game against the Florida Marlins on July 18 in the third inning and didn't return to the lineup.
It's been a rough year for Guillen, who began spring training recuperating from offseason shoulder surgery and then injured his wrist. He also missed four games in April with a strained chest muscle and then went on the disabled list in May with a hamstring injury.
He entered the season with a .276 career batting average, but he hit only .216 in 2006, with nine homers and 40 RBI in 241 at-bats for last-place Washington.
Bowden said Guillen will head home to Miami after the operation to begin rehabilitation work.