Here’s the medical update press release from the Mets…
FLUSHING, N.Y., January 13, 2010 -- Mets centerfielder Carlos Beltran had worsening of osteoarthritis of the right knee during the offseason. He had not been experiencing pain following the conclusion of the season and into his early offseason conditioning. The symptoms returned to the point where pre-spring training conditioning became too painful.
He elected to undergo arthroscopic clean out of the arthritic area of his knee by Beltran's personal physician Dr. Richard Steadman today in Colorado. He is anticipated to return to baseball activities in 12 weeks.
Emphasis mine.