CLEVELAND - Prior to tipoff, coach Jim O'Brien had spoken about the importance of playing with energy during a stretch some might call the dog days of the regular season. For a team playing its ninth game in 17 nights, and doing so amid empty seats at Gund Arena, summoning energy was not an easy task. But when the Celtics needed to raise the level of their play with the game on the line in the fourth quarter last night, they did.