The bad news comes a day after the team announced that lefty ace Johan Santana would go under the knife to correct a problem in his pitching shoulder
Beltran says he felt pain in the knee after stepping hard at first base during his first at-bat against Philadelphia Friday night, but he was able to finish the game. He is not certain if he will play Sunday.
Beltran played with tendinitis in both knees in 2006 and '07.
The center fielder was told by doctors after having offseason surgery on the right knee that he might develop the condition. Doctors noted it's natural to put more stress on the healthy leg while rebuilding strength in the injured one.
Angel Pagan was in center field for Saturday's game against the Phillies.