The Celtics have the best defense. Even without Robert Williams, who is expected to miss at least the start of the series, they have a long list of formidable options to throw at the Nets. Marcus Smart, Jayson Tatum and Jaylen Brown all are long, rangy and disruptive. Horford, despite his age and position, has been up there among the most versatile bigs this season. The Celtics believe everyone they play can defend at a high level, and they won’t let a player handle his duties alone. They will send smart help. They will call for occasional double-teams. They are aggressive enough, big enough and intelligent enough to cause issues — even, they hope, against Durant and Irving. “Everybody’s got to do their part,” Jaylen Brown said. “It’s gonna be a great challenge. Those are two offensive juggernauts over there. Like, what they do with the ball is special. So it’s gonna take everybody. It’s not Al Horford by himself (on a switch), or JT or Jaylen in an iso matchup. We gotta play team basketball if we want to win.”