NBA

Volatile Smith proving to be indispensable piece to Knicks’ puzzle

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.

Smith has taken mounds of pressure off Anthony, whose equilibrium was altered Sunday. Smith was back home watching the game in New York. Anthony’s 10-of-35 brickfest (including 0-for-7 from 3-point range) was his most selfish game this season. Anthony said he has respect for Smith’s scoring and has no problems sharing the spotlight.

“Of course it put a damper into a lot of our plans,’’ Anthony said.

Smith’s dark side is tough to explain. He was raised middle class, in Lakewood, N.J., by two attentive parents in Earl and Ida Smith. Earl Smith, a former Monmouth guard, taught J.R. everything about shooting technique. Earl also doesn’t have a tattoo on his body.

Some in the league wonder if Smith’s tough-guy behavior is an act to relate to the NBA’s hip-hop crowd. He is such a vital cog to the machine that the Knicks organization accepts Smith for who he is and desperately wants to re-sign him.

As much as Mike Woodson has toned him down and got him to stop wearing “sagging jeans’’ to the arena, the Knicks coach hasn’t completed the job. Smith still is the most likeliest Knick to start a brawl in these playoffs.

Woodson loves Smith’s toughness, but there’s a difference between being tough and being a hooligan. Anthony loves Smith’s ability as one of the few Knicks who can carry the scoring load on his back, giving him a breather. When they are on the court as a tandem, the Knicks are a plus-7.4 points per 100 possessions.

Smith is indispensable. He also is a powder keg.

“J.R. is a big-time scorer for us, makes a lot of tough shots for us,’’ Iman Shumpert said. “He’s another guy who plays the passing lanes and rebounds. We’re losing a lot not having J.R. We got to figure it out.’’

Hopefully the Knicks won’t have to figure it out without Smith against Miami in late May.

marc.berman@nypost.com