Josh Hart was sore. Extremely sore. But nothing was going to stop him from making his season debut Friday night against the Boston Celtics.

The Knicks forward had been sidelined for three weeks after suffering lower back spasms just seven minutes into the Oct. 2 preseason opener in Abu Dhabi. He missed the remainder of New York’s exhibition slate and the regular-season opener against the Cleveland Cavaliers at Madison Square Garden on Wednesday.

“You don’t really know how often you use your back until you hurt your back,” Hart said at his locker ahead of tipoff Friday. “Last week I was feeling good, kinda hitting all my checkmarks, and then I tried to go through doing live contact stuff and it was just like — ‘yeah, nah. It’s not there. It’s not ready yet.’

“It’s frustrating ‘cause

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