“Plummet for Porter.’’ That could turn into the unofficial motto for the 2017-18 Knicks sooner than later. If you haven’t heard of him yet, that’s Missouri 6-foot-10 freshman combo forward Michael Porter Jr., expected to be the top pick in the 2018 NBA Draft. “He’s a game-changer,’’ one NBA executive told The Post. “This year was a good draft, not sure yet there was a game-changer.’’ Nobody from the Knicks organization — especially coach Jeff Hornacek — will use the word ‘tanking’ about next season, but if it smells like former Sixers general managers Sam Hinkie’s signature “Process,’’ it likely is. According to multiple sources who have spoken to members of the organization, Knicks acting president Steve Mills is all in for a youth movement, except at point guard where they are attempting to find a veteran mentor for 18-year-old Frenchman Frank Ntilikina. “The focus is on young players but they still need some veteran presence,’’ said one team executive who has spoken to Knicks officials. “They’re waiting to see where there are good young players that would take shorter deals. The focus is definitely on building around young guys.’’ The timing isn’t great — four straight years out of the playoffs before contemplating a complete rebuild. But they are stuck in between, with a share of veterans but not a roster good enough to do anything beyond an eighth seed at very best. Hence, the attempt to trade Carmelo Anthony, 33, even after Phil Jackson’s ouster. Dormant in free agency across the first four days, Mills even let their 29-year-old backup shooting guard Justin Holiday skip away on a two-year, $9 million pact. The Knicks didn’t have to eat 2017 cap space to re-sign him because of the Bird exception. But Mills didn’t want Holiday stealing away 2018 space.