Beltran has yet to miss a game due to the injury, which doctors have compared with the calf tendinitis that has kept Jose Reyes out of the Mets' lineup for four straight contests.
"I want to make sure what I have doesn't get worse," Beltran said. "Mentally, I know something is wrong."
Beltran initially injured the knee during last week's series in Los Angeles, although he can't remember an exact play that caused the pain. He served as the Mets' designated hitter for three games in Boston before returning to the outfield on Monday against Washington.
Tim Britton is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.