The Mets aren’t going to be able to decide when Johan Santana is ready to pitch. Only Santana — and his fragile left shoulder — can do that. “Whatever the schedule is, Johan is ultimately the final arbiter, whether that’s articulated or not,” Mets general manager Sandy Alderson said yesterday. “Because he’s the only one who can judge how he really feels.” Pitching coach Dan Warthen said Saturday the team would leave the next steps in Santana’s hands after it gave him the day off from throwing. “We need to get him ready,” Alderson said. “His condition will dictate his schedule at this point.” With the likelihood of his availability for Opening Day decreasing by the day, the Mets are determined not to rush Santana. They also don’t want a repeat of last week, when he took the mound for an impromptu bullpen session. “I don’t concern myself with any sort of emotional response,” Alderson said. “It’s all about, ‘Let’s get [Santana] ready on the appropriate schedule.’ ” Santana was unscathed following the bullpen session, but he got the Mets’ attention and he has yet to throw another one. Regardless of the plans the organization may come up with during his rehab, they won’t be as important as what Santana tells them. “Periodic judgments rather than daily judgments, I think, are more valid and allow us to focus on more details,” Alderson said.