The Rangers have amassed the best record in the NHL through 80 games by mixing and matching on defense behind the redoubtable Dan Girardi-Ryan McDonagh first pair that is to this regular-season conference champions what the Zdeno Chara-Dennis Seidenberg tandem was to the 2011 Stanley Cup winning Bruins and the Duncan Keith-Brent Seabrook unit was to the 2010 Cup champion Blackhawks. But in order to achieve ultimate success in the playoffs, the Blueshirts, who face the Penguins tonight before Saturday's finale at the Garden against the Capitals, will need a strong, reliable second pair on which to lean. Girardi and McDonagh can't play 30 minutes a night into the middle of June, no matter how much they're willing.