J.R. Smith’s tweet following Game 1 against Boston 10 days ago said, “#Happy4/20.” April 20 is the unofficial holiday for pot-smokers. As father of a high-school junior and a college freshman, I prefer my kids don’t get that sort of encouragement from the winner of the NBA’s Sixth Man Award.

The league was aware of the “#Happy4/20’’ tweet, but could do nothing about it. It’s a date. The Players Association would have had a legal field day appealing.

The league was able to do something about Smith’s right elbow to Jason Terry’s jaw Friday in Boston in an assault disguised as a basketball play. Commissioner David Stern knocked Smith out of a playoff game like Smith knocked Terry to the floor.

Those who watch Smith knew better not to believe Smith’s alibi of trying to create space. He’s sneaky in delivering his cheap-shots through the guise of basketball moves. We’ve seen it before. He has been ejected three times this season.

If the latest elbow was an accident, Smith would have bent over to see if Terry was hurt. Instead, Smith marched away with no remorse. Terry had been hounding him, slapping hard at the ball, triggering the bad J.R.

Unfortunately, Smith’s behavior matters too much. It won’t cost the Knicks the Boston series, as they should take out the aging Cetlics tomorrow in Game 5 at the Garden with Smith back in uniform and Carmelo Anthony back to himself.

Anthony looked like he missed Smith like a cat misses his food dish in Game 4.

That is the alarming truism about the 2012-13 Knicks. Smith is not just the league’s Sixth Man Award winner, but the Knicks’ most indispensable player. He’s taken for granted, along with his career-high 18.1 points and menacing rebounding.

When Anthony missed games this season, the Knicks managed to win some, with Smith playing hero. Smith beat Phoenix with a buzzer-beater with Anthony out. Smith beat Charlotte with a buzzer-beater with Anthony out the second half with a cut finger. The Knicks also won in Miami sans Anthony.

Smith played the season’s first 80 games before resting the final two with Anthony. Sunday became the first time Anthony was on the floor all season without his wing man. And Anthony failed, proving part of his MVP-caliber season is having one guy he counts on as much as himself.