When Jeremy Lin returned on Monday in Houston after missing 21 games with a hamstring injury, he gave the Nets something they’d been missing. On Wednesday they were back to missing him.

The star point guard had tightness in his lower back and was ruled out shortly before tipoff against the Lakers at Barclays Center. He is day-to-day and the hope is he will be able to play in Friday’s game at Orlando.

Before the Lin injury, coach Kenny Atkinson said Jordan Hamilton would be back after missing two games with migraine headaches — which would have put the roster at full strength for the first time all season.

“I really don’t know,” Atkinson said when asked if Lin would play Friday. “Who knows how these things happen? We’ll evaluate him tomorrow.”

Lin looked very smooth in his return from a hamstring injury and it was a big lift for the Nets, who played their best during the 20 minutes he was on the floor against the Rockets. He finished with 10 points and seven assists, engineered a fourth-quarter comeback that just fell short and led all Nets with a plus-17 for the contest.

The hope was that he would play even more minutes on Wednesday night.

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“The game goes so fast, you really need kind of another coach on the court and he’s that guy with his experience and understanding of what we’re trying to do,” coach Kenny Atkinson said. “It makes it easier on me and it makes it easier on his teammates . . . knowing where to get guys the ball. Defensively also, you watch the tape and he brings a certain energy, a certain intelligence, a certain aggressiveness.

“He engages in everything we do. It’s big.”

NBA fines Walton

Lakers coach Luke Walton got hit with a $15,000 fine from the NBA on Wednesday for his actions in Monday’s loss to Sacramento. The league statement said the fine was for “verbally abusing a game official and not leaving the court in a timely manner after his ejection.”

Walton screamed at officials after the was no foul called on a play where Julius Randle was thrown to the ground. “If our guys aren’t getting what I feel is ‘treated right’ then I’m going to stand up for them,” Walton said.