Tommy Heinsohn used to tell the story of how the 1963-64 Celtics used Bob Cousy’s retirement after the previous season as motivation. While The Cooz was clearly a cherished member of the club, the competitive C’s had the internal mantra of proving to themselves they could win a championship without him. And they did. In the wake of losing Rajon Rondo with a torn ACL, one wonders whether the current Celts, who canceled yesterday’s practice because of the storm, aren’t trying to make a point to themselves and the basketball world as they seek their seventh straight win tonight at the Garden against Denver. “I mean, it ain’t something that’s been talked about aloud, but I’m sure guys have certain motivations,” said Paul Pierce, whose club also suffered a blow when rookie banger Jared Sullinger went out with season-ending back surgery. “But the big thing with the motivation is that they have opportunity now. I think that’s what’s happening right now. “Given the opportunity that’s at hand, certain guys are taking advantage of it and looking pretty good doing it.”