Everything old is new again. Or, in this particular case, Blue again. That’s because, after an interruption of more than one season, the Rangers have at least temporarily reunited the blue-line shutdown pair of Marc Staal and Dan Girardi that was a staple throughout much of 2009-10 and essentially all of 2010-11. It was only when Staal reported to last season’s camp with post-concussion symptoms, which kept him sidelined for the first 36 games of 2011-12, that coach John Tortorella moved Ryan McDonagh to Girardi’s left to form a new top defense pair that remained intact even after No. 18 returned and through the initial stages of this season. But a week ago tonight, Staal and Girardi skated as a tandem at Montreal. And they were together again at the Garden against Tampa Bay on Thursday while McDonagh — who was sidelined Tuesday against Winnipeg — was paired primarily with Anton Stralman for the second straight game while healthy. Tortorella cited both a return to full form by Staal and a dip in performance by McDonagh to explain the switch that likely will remain in place for the start of tomorrow night’s match at the Garden against the Sabres, though nothing is guaranteed. “Mac has been fighting it a bit; he’s been fighting his reads,” said Tortorella, whose team was off yesterday in the wake of Thursday’s Nashterful 4-1 victory over the Lightning. “I want to take a step back and take some pressure off him from facing the top line.”