Whatever magic last Monday's cortisone shot worked on Ramon Castro apparently ran out yesterday as the Mets placed him on the disabled list with lower back arthritis. Two days earlier, in Pittsburgh, Castro thought for sure he would be ready to return for the weekend series against the Nationals. By Friday, however, Castro had stiffened up again. He couldn't run and trying to hit in the cages wasn't much better. "It was the right decision," Castro said from the clubhouse couch yesterday afternoon. "When I run, I feel it. I can hit, but it's not me." The move is retroactive to Aug. 13, so the Mets gained another five days with Castro by playing it safe. Castro left the team yesterday and will begin his rehab Monday in Port St. Lucie. One person happy to have Castro's status resolved was Sandy Alomar Jr., who finally was allowed to join the team yesterday. Earlier this week, the Mets flew Alomar to Pittsburgh in the event Castro's health did not improve, but sent him back to New Orleans.