By Michael Pina How Boston chooses to guard Milwaukee’s star point guard could decide the series. The Bucks won Game 1 of the Eastern Conference semifinals with defense. Trying to navigate the impenetrable Brook Lopez—whose back surgery might as well have never happened—the Celtics shot 45.5% at the rim and a whopping 44% of all their shots were non-corner threes (a number only topped twice this season: Jan. 6 against the paint-packing Knicks, and last month against … the Bucks). Milwaukee picked ballhandlers up 90 feet from the rim, invaded personal space and poked balls loose when they it wa…