Rendon was hurt March 9 diving for a ball in the field. The Nationals expected Rendon to return within a week, but pain lingered. He was diagnosed with a sprained medial collateral ligament and was examined this week by Drs. James Andrews and Steven Singleton. He will need rehabilitation time.
''He's missed so much time, it will be a fairly lengthy one, I'm sure, to get him back in shape,'' manager Matt Williams said Wednesday.
Rendon hit .287 with 21 homers and 83 RBIs last year but has been limited to nine exhibition at-bats.
''The doctors are confident there is no long-term issue here,'' Williams said.
Williams also expects outfielder Jayson Werth to begin the season on the DL. Werth had shoulder surgery Jan. 9 and has yet to play in a big league exhibition game, although he has started appearing in minor league games.
''If he gets to the point where he says, `I'm good to go,' we trust him to let us know the truth,'' Williams said, ''but as of right now, if we were to start today he would not be on the opening day roster.''
Rendon and Werth are scheduled to leave with the team when the Nationals travel to Washington for an exhibition against the New York Yankees on Saturday. Washington opens its season on Monday against the New York Mets.
Williams expects both players to return to Florida around April 9, around the time minor league seasons start.
Williams said closer Drew Storen, who left Monday's game because of a blister on his toe, should resume throwing in coming days and avoid a DL stint.