Something had to give for the Islanders, and it turned to be the inevitable, long-awaited demotion of Rick DiPietro. The Islanders’ oft-injured goalie was placed on waivers yesterday, and when he clears by noon today, he will be assigned to the Bridgeport Sound Tigers of the AHL, where his attempt to relearn his craft can begin in earnest. Having played just 47 games the past four seasons due to a litany of physical ailments, DiPietro looked lost in three games this year, going 0-3-0 with a 4.10 goals-against average and a .855 save percentage. “It’s just a situation where he wasn’t getting much playing time,” general manager Garth Snow told The Post last night from Buffalo, where the Islanders will play the Sabres tonight. “Just getting reps in practice, I felt he needed play in more game situations. It’s a good opportunity for him to go and play games in Bridgeport and try to find the formula he had when he was an All Star [in 2008].” When DiPietro clears waivers — rest assured, no team will claim him due to the 15-year, $67.5 million contract he signed in 2006 — the Islanders will recall 22-year-old Kevin Poulin to back up starter Evgeni Nabokov. Poulin was set to start last night for the Sound Tigers in Springfield, Mass., and if he can’t make it to Buffalo for tonight’s game, DiPietro would stay with the team for emergency purposes. “[DiPietro is] obviously highly motivated and he takes a lot of pride in being a New York Islander,” Snow said about the 31-year-old from Winthrop, Mass., who was drafted by the Islanders No. 1 overall in 2000. “At the end of day, we just want what’s best for him.”