The temperature drops and the light fades on the outskirts of Doc Rivers’ radar. Ask Courtney Lee, who has slid off the grid. Ask the seldom-used big man tandem of Chris Wilcox and Shavlik Randolph. Ask Terrence Williams, who abruptly became part of the solution during the Celtics’ Game 5 win over the Knicks Wednesday night at Madison Square Garden — a win that cut New York’s series edge to 3-2, with Game 6 tonight at TD Garden. The Celtics, still breathing in the Eastern Conference quarterfinals, had become especially vulnerable to New York’s swarming traps and fullcourt pressure. When Rivers started the series by lamenting his lack of a true point guard, it was with the thought that Williams was more project than ready-made playmaker. None of Williams’ previous teams — not the Nets, Rockets or Kings — had ever taken him seriously as a point guard. But Rivers, in a tight spot, placed the swingman in the role on one of the league’s brightest stages Wednesday night. Perhaps the most impressive part of Williams’ sometimes wobbly performance was that he played 17 turnover-free minutes. He swung the ball and, when necessary, scored, including a third-quarter dunk after the Knicks defense gave him a matador’s welcome to the lane. Right now the Knicks don’t really care if the light-shooting Williams scores. And he’s not foolish enough to take that attitude as an open invitation to attack the rim.