Nationals manager Matt Williams told reporters Wednesday that a third doctor's opinion on Rendon's knee confirmed the sprain. Williams said that Rendon will continue rehab but won't be ready in time for Opening Day.
Rendon met with Dr. Steven Singleton in Vail, Colorado, on Tuesday for a third opinion on his injured left knee after Dr. James Andrews on Monday confirmed the team's initial diagnosis of a sprained medial collateral ligament.
Rendon was hurt making a diving stop in a game against Atlanta on March 9. He finished fifth in the National League MVP voting last season after hitting .287 with 21 home runs and 83 RBIs.
Washington has used Danny Espinosa and Ian Stewart at third since Rendon's injury. However, with the uncertainty of when Rendon will be back, Yunel Escobar has volunteered to play there, with Espinosa moving back to second.
Also Tuesday, the Nationals agreed to pay left-handed reliever Rich Hill a $100,000 retention bonus. In camp on a minor league contract, Hill has allowed two runs over 6 1/3 innings in seven spring training appearances with eight strikeouts and three walks.
The Associated Press contributed to this report.