It’s not Carmelo Anthony’s left shoulder. It’s his brain.

Anthony, off to a mediocre start after three games, went introspective at Monday’s practice, saying he hasn’t played with enough of a cold-blooded scoring mentality, that he has been “passive.’’

“For myself, maybe I’m second-guessing myself,’’ said Anthony, who is shooting 37.7 percent during the Knicks’ 1-2 start and averaging four turnovers. “Maybe I’m a little bit passive out there, trying to do things that’s out of the norm, trying to make people better at the wrong time. That’s where I’m second-guessing myself, second-guessing my shot. I got to get out of that.’’

Anthony shot just 37.5 percent in preseason, after which he vowed he’d be back on target once the regular season dawned. He’ll have another chance Tuesday to find his errant jump shot, when the 1-2 Bobcats visit the Garden in the first matchup of a home-and-home.

It’s worth noting Anthony’s best two seasons came after Olympic years, in 2008 and 2012, when he played all offseason. This summer he rehabbed a torn shoulder and wasn’t able to pick up a basketball in June and for most of July. Anthony said the shoulder, however, is fine, despite needing to ice it after virtually every game.

“I don’t want to make no excuses,’’ Anthony said when asked about his summer rehab. “I got to get better. I got to do better. I got to put the ball in the hole. That’s what I do best. And I got to do that. No need for me to take that away from myself when I know I can do that. When I do that, we’re a helluva team.’’

Indeed, Anthony won the scoring title last season (28.7) and finished third in the MVP balloting — a season in which the Knicks started 6-0 and never dipped below .500. He’s still rebounding, averaging 11 boards. But his late-game offense has been missing.

In Chicago, he missed his last five shots, including what would have been the game-winner, and Sunday he was held scoreless for the game’s final 5:55. Derrick Rose stole Anthony’s thunder in Chicago, and Kevin Love and Kevin Martin outclassed him Sunday in Minnesota’s victory. It’s also a concern whether the pressure of impending free agency, and the criticism he received for making comments about it, has hurt his psyche.

“Scoring the basketball is normally our MO and it’s gotten away from us in the fourth quarter,’’ Anthony said. “We got to figure that out. Keep shooting the ball. Believe in myself and have confidence in myself to take those shots and make those shots. We’ll figure it out.’’

Help to “figure it out” is expected to come Sunday against the mighty Spurs, when J.R. Smith can return from his five-game suspension for violating the league’s drug policy.

When Anthony was at his best, he had his trusty former Denver wingman.

“We all miss J.R.,’’ said Anthony, averaging an empty 21 points. “We all know what he brings to our team, the spark he brings to our team. We miss that part.’’

Anthony believes the Knicks as a whole are being tentative, assigning the team’s uncertainty as the cause for their glut of turnovers — 17.7 per game.

The hallmark of last season’s club was taking care of the ball, leading the league in fewest turnovers at 12 per game.

“We’re thinking too much,’’ Anthony said.“We’re out there thinking too much and second-guessing ourself out there. When you do that, you’re going to make mistakes and turn the basketball over. It’s different when we’re just having fun and playing basketball out there, going with our gut and first instinct. It’s night and day out there.

“It’s frustrating but we can correct that. We’re second-guessing everything out there on the court.’’

Coach Mike Woodson hasn’t established a firm starting lineup, either. He benched Andrea Bargnani for the season-opener, going with the two-point guard alignment of Raymond Felton and Pablo Prigioni from last season, with Anthony at the power forward.

Woodson is set to go with Bargnani in the jumbo frontcourt for the third straight game Tuesday, even though the team is 0-2 with Anthony at small forward. Woodson has been hesitant closing games with Bargnani.

Anthony wouldn’t talk on that topic.

“Things aren’t going our way, so it’s easy to point this out or that out,’’ he said. “Normally at the end of the game, J.R. would be out there with us. It’s another missing piece. No need to panic. Three games into the season. We’re trying to figure it out on the go.’’