The Celtics may have already blown their best shot. With the Knicks vulnerable Saturday in Game 1, the Celtics may have lost their only opportunity to seize momentum in the first-round series. Evidence suggests the Knicks are only going to get better and it’s only going to get tougher for the Celtics as the series resumes tomorrow with Game 2 at the Garden. The Celtics gave the Knicks a good scare, but the Knicks’ record-setting fourth-quarter defense sealed the Game 1 win, 85-78, and made them again resemble a club with championship mettle. If defense wins titles, the Knicks, reputed more for their record-setting 3-point shooting and league-best turnover rate, are on their way. The Heat should look over their shoulders for a May slugfest. “It’s what good teams are supposed to do,’’ coach Mike Woodson said in yesterday’s conference call regarding their playoff-record fourth-quarter defense in holding the Celtics to eight points. “We’ve been one of the best fourth-quarter defensive teams in the league [ranked second]. That’s the time you have to step up and make plays. I thought [Saturday’s] game, we were probably more solid than we’ve ever been all season in terms of locking in and rotating.’’