It’s the existential question that will hover over the Knicks until proven otherwise. Can a team win a championship with a small, less-athletic guard — like the Knicks with Jalen Brunson — as their best player? “Tough. It’s tough. Even if you just took out the ‘not as athletic’ component, and you just said ‘at his size as your highest-volume shot-taker,’ just look historically how many teams have done that,” former NBA guard Tim Legler, now ESPN’s top NBA analyst, told The Post. “And the teams