The biggest problem for the Knicks regarding rookie point guard Frank Ntilikina simply has been getting him on the floor. It won’t happen Tuesday night in Boston, either. Ntilikina, who missed summer league with a sore knee, all but one preseason game with a bruised knee then sat the second game of the regular season with a sprained ankle, will be out against the Celtics, coach Jeff Hornacek said Monday. Ntilikina was a virtual inactive at practice because of the ankle. “He did the shooting part of it but didn’t do any live stuff,” Hornacek said. “I don’t think he’ll be out there [Tuesday].” Ntilikina, the eighth pick in the draft, is a key component of the Knicks’ rebuild, targeted for point guard, a supremely important spot in the modern NBA. The Knicks could be reminded of that, perhaps painfully, in Boston by Kyrie Irving. “You have to look at the league and how a lot of teams are going smaller,” Hornacek said. “It’s the penetration and getting the floor spread. When you have a point guard like Kyrie, who can do that, then the other guys are free to cut and move. Penetration is big in this league, whether you get it through a pass or a dribble, but a lot of these point guards in the league can break you down.” And cause havoc. “It’s a total team defense that you have to have against these guys,” Hornacek said.