Dr. James Andrews on Monday confirmed the team's initial diagnosis of a sprained medial collateral ligament, and Rendon was to be examined by Singleton on Tuesday.
"We're just trying to make sure we cover all bases and everybody gets a chance to take a look at it and develop a course of action from today on," Nationals manager Matt Williams said.
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 also 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.