You see them against the Detroit Pistons or the Indiana Pacers recently and you think, why not? You see them against the Cleveland Cavaliers and the Memphis Grizzlies recently and you think, why bother?
Can the Celtics turn it on and make a serious run at the NBA championship? They are saying it's possible, which is what you'd expect them to say. History, both recent and in the more distant past, tells us it's possible -- but highly unlikely.