One play doesn't decide a game, or so every coach on the planet will tell you, but we just cannot talk about anything in Game 1 of the Eastern Conference finals until we talk about the Pacers not fouling the Celtics with a 3-point lead and less than 10 seconds to play. Instead, Indiana allowed Jaylen Brown to catch in the corner and raise up for the tying 3. 

Take a look:

Now, both Rick Carlisle and Tyrese Haliburton said the intent was to foul on the play. But with the way Brown caught the pass already faced up to the basket with the threat of pulling the trigger immediately, Pascal Siakam made the quick decision to not risk fouling on a 3-point attempt. 

"Yes [there was intruction to foul]," Carlisle said. "But [Brown] caught the ball and was faced up, so Pascal decided to lay off, which was... I understand. That's probably the right decision, you don't want to give up a four-point play. Look, a lot of things had to go wrong for us and right for them. They did. We have to own it."

That is all true. If the Pacers don't turn the ball over on the preceding inbound pass, the game is probably over before Brown ever has a chance to make this shot. And yes, once Brown caught the ball in the faced-up position, Siakam, trailing the play, would've been playing with fire reaching in for a quick hack. 

In fact, Siakam executed a terrific close out to pin Brown in the coffin corner, nearly behind the backboard, considering his lag time, and he was careful to clear his feet of Brown's landing space. He could've gotten a hand up on the actual shot, but at that point he was decidedly, and correctly, in don't-foul mode, and he simply forced Brown to make a supremely tough shot. Siakam made the correct call in that position. 

The mistake was allowing Brown to catch the ball in that faced-up position in the first place, which was only possible because Siakam was trailing behind off the misdirection screen action that Boston ran. 

Indiana's entire defensive alignment and subsequent execution should've been devoted to nothing except guarding the 3-point line. Matchups don't matter. Anything inside the arc doesn't matter. Closest man to a shooter takes the assignment. Switch everything. What's the risk? That someone slips for an easy layup? Cool. Two-pointers don't hurt you. Don't want a small guy guarding a big guy? Who cares. You're fouling anyway. 

Watch the 3-point clip again below, this time with a bit more lead time to see the play developing, and you will notice that T.J. McConnell, initially defending Derrick White, had a perfect angle to see Siakam getting tangled up and was in perfect position to switch out to Brown. 

If McConnell switches he can beat Brown to the spot and either thwart a pass altogether or be in position to foul immediately on the catch. But McConnell stuck with White, forcing Siakam to chase Brown from a step behind, and by the time he arrived it was too much of a bang-bang situation to risk making contact. 

"As soon as I got to him, I was a little late because of the screen, he was going up so I didn't want to do it, Siakam said of the decision not to foul. "When I thought I was going to do it, it was a little too late… it was just a judgment call. I felt like he was going through the motion, he had a pump fake. I didn't want to foul then. And it was a tough shot, I was in front of him. Maybe I could have contested it better but it was just a tough play."

It was a tough shot. Give Brown credit for knocking it down. But these guys are pros. They make great shots look routine all the time. Tyrese Maxey did it to the Knicks in Game 5 when the Knicks should've closed that series out. It happens more than you think. More than you see a 4-point play, I'll tell you that much. 

You just cannot give NBA shooters the opportunity to make a game-tying shot if you have any way to avoid it. For some reason, you see more and more coaches opting not to foul in these situations. They are just so paranoid about the worst-case scenario, which is an extremely unlikely outcome, that they lose total track of basic probabilities. That's a mistake. LeBron James, who published the tweet below immediately after Brown's game-tying shot, certainly knows the deal

OK, so Carlisle ordered the foul. But that's only part of the job. The other part was to ensure his team was in proper position to commit the foul. If your guys aren't in position, the instruction to foul becomes moot. 

Frankly, the Pacers shouldn't have been hugged up so tight on Boston players inside the arc from the start. They should've basically been face-guarding the 3-point line rather than any single player. All of Boston's maneuvering inside the arc, which was only meant to cause confusion as a two-pointer didn't matter one bit, should've been ignored. And everyone should've been under clear switching orders. Udonis Haslem said as much on NBA TV right after the game, and he's right. Switching should be, and typically is, the standard defensive procedure in this situation, and it should've made unequivocally clear by Carlisle. 

This is not the first time Carlisle has been burned in this situation during these playoffs. In Game 3 of the Pacers' first-round series vs. Milwaukee, Carlisle, again with a 3-point lead, elected the let Milwaukee sharpshooter Khris Middleton mosey from one side of the court to the other, with multiple opportunities to commit a non-shooting foul, for a game-tying shot in the closing seconds. 

The Pacers escaped that game with a win in overtime, but they weren't so lucky on Tuesday. Everyone will talk about how the Pacers should've fouled, but I want to be clear one final time. Siakam made the right call to play it safe from the position in which he found himself. He just never should've been in that position. The Pacers should've switched. 

Now, I don't know if Carlisle instructed his guys not to switch, as they tried to avoid it for most of the game, or if McConnell just made a bad read, but either way, that's where the mistake happened. And it cost the Pacers and opportunity to grab an unexpected hold of a series in which they are now fighting uphill.