Washington Nationals outfielder Adam Eaton has undergone surgery to repair his torn left anterior cruciate ligament and started a rehabilitation program Wednesday that is expected to last six to nine months.

The injury occurred April 28 in a game against the New York Mets, when Eaton stepped on first base awkwardly while beating out an infield hit.

Eaton had surgery Tuesday, Nationals general manager Mike Rizzo said.

"It went well," Rizzo said. "They didn't find anything that they didn't anticipate."

Surgeons also repaired some other ligament damage and the meniscus in the knee, Rizzo said Wednesday.

Eaton's rehabilitation began with massage and range of motion work. It will be around a month before he can put weight on the knee.

"In four weeks he'll ramp it up," Rizzo said. "He's tolerating the pain and on his way to getting better."

Eaton was obtained in December from the Chicago White Sox for three pitching prospects. With a lofty .362 on-base percentage from 2014-16, the 28-year-old was expected to bolster the offence and provide stability in centre field.

Eaton was batting .287 with two homers, 13 RBIs, 24 runs scored and a .393 on-base percentage in 23 games when the injury occurred.

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