“What’s up, Doc?”

The Celtics enter Game 5 of their series against the Philadelphia 76ers coming off an Elmer Fudd-like performance Sunday at the end of regulation and overtime.

The Celtics were hunting “wabbits” and Doc Rivers had all the angles covered. What was daffy about the end of Game 4 was not the lack of a timeout call at the end of regulation or overtime. No, that is the norm. Inexplicably somehow.

No, what really bugs us about the end of Game 4 was this troubling fact pattern: Jayson Tatum had the ball in the lane at the end of regulation with 4 seconds to play and did not shoot it. He had the ball and was headed for the rim at the end of overtime with less than 3 seconds to play and, once again, chose not to shoot.

Part of this was by design on Tatum’s part. He saw Marcus Smart open for uncontested threes. He gave up the contested shot. He went by the book.

It might be time to turn the page.

As a rule, coaches preach “unselfishness” at every level starting at the Fidelity House in Arlington. That is the sacred ground where Pat Connaughton and yours truly began their basketball careers. Pat went on to win an NBA title. My career was cut short by poor eyesight and an appalling lack of talent in the Brackett School auditorium/gym.

We keep being told that Tatum is HIM (grammatical questions notwithstanding). But to be HIM. YOU must not only want to take the shot. You must take the shot. And you must make the shot. Especially during the playoffs.

Don’t get too excited by Tatum’s stat sheet or his two-way play down the stretch. He was non-existent in the first half, missed two free throws in the fourth quarter, and bailed on two potential game-winners.

The Celtics showed zero urgency when bringing the ball up court after Harden’s final shot in overtime. If the reason for not using a timeout is because you don’t want to give the opposition time to set its defense, why stroll up court?

Pat Riley called a timeout in Game 4 of the 1987 NBA Finals to set up Magic’s Baby Skyhook that basically ended Boston’s dynastic run of the mid-1980s. Magic slapped the Celtics on Twitter for not using theirs Sunday.

Timeouts. Good enough for Magic and Kareem.

Tatum and Smart, not so much.

But there is another part to the equation that resulted in Sunday’s loss. And that is Rivers.

Joe Mazzulla has been schooled by Rivers. While the Sixers boast the ailing current and a mercurial former MVP on their roster, Doc has gotten the most out of his otherwise unheralded squad. Mazzulla is two losses in this series from earning the permanent moniker of “Clueless Joe.”

Harden wrecked the Celtics in Game 1 and then did it again on Sunday. Mayor Wu should reopen the Combat Zone immediately. That might be the only thing to keep Harden under 40 points if the series returns to Boston for a Game 7.

Jaylen Brown copped to the mental lapse that left Harden open to hit the game-winner. Good for Brown. But he took three shots in the fourth quarter and none in overtime.

Are we supposed to believe that, too, was a mental lapse? If that’s the case, the Celtics would be much better off using any max deal elsewhere.

A more reasonable explanation for Tatum’s inability to attempt a potential game-winner twice, and Brown’s three shots over the most crucial 17 minutes of the season thus far, might be the Sixers’ defense.

The Celtics had a five-point lead with 2:04 to play in regulation. It was gone 73 seconds later. During that span Boston took one shot – a Marcus Smart chuck from Cambridge.

Same old story. No one wanted to close.

Combine that with Rivers’ willingness and determination that his team would not be beaten by either Brown or Tatum when the plays mattered the most,  and you had a 116-115 loss.

Smart was open all game. By design. The Sixers gave him the open three at the end of regulation. And, unfortunately, he missed it. The Sixers gave him the open three at the end of overtime. Unfortunately, it was too late.

Just like the Doc(tor) ordered.

Two calculated moves by Rivers that paid off with an overtime win, a tied series, and another game guaranteed at the Wells Fargo Center.

Rivers made sure Brown was smothered whenever he had the ball. Tatum, too, faced the same sort of defense. He needed to plant his right arm in the chest of Philly’s Tyrese Maxey to clear space for the three-ball that gave Boston its final lead of the game.

The non-call on that play gave Rivers an opening. Doc laid into both the refs and an unwitting reporter who asked him if Tatum had committed an offensive foul.

“Jayson Tatum’s three was awful that that wasn’t called,” Rivers said after the game. “It was a push-off. And the reason that it bugged me was because at the end of the game, there were these touch fouls. Jayson Tatum has a 360 foul. Marcus Smart and (P.J. Tucker) gets tangled up. Touch foul. So if we’re going to call it that way. You have to call (the Tatum foul).”

Rivers has now put the refs on notice and planted another seed of doubt in Tatum’s psyche. An area that has been overrun by a bountiful harvest of second-guessing and lack of killer instinct.

While Doc is playing 5D chess with the refs, Mazzulla was still holding on to his timeouts while offering the following explanation of his team’s endgame self-destruction.

“It was great execution.”

The Celtics need much better than “great” in Game 5.

Bill Speros (@RealOBF and @BillSperos) can be reached at bsperos1@gmail.com.