Forty-eight hours removed from a heart-wrenching Game 3 loss, the Boston Celtics improbably — and inexcusably — failed to match the intensity of the Toronto Raptors in Saturday’s Game 4.

Suddenly, a series that Boston once seemingly had a vice grip on has slipped from the Celtics' fingers and Toronto has all the momentum in what now amounts to a best-of-three.

Boston’s lethargy on Saturday is hard to wrap one’s head around. The Celtics played with purpose in spurts but often couldn’t get out of their own way. Boston missed 28 3-pointers, turned the ball over 14 times, and watched the Raptors generate 24 second-chance points while shooting 9-of-10 overall on those extra possessions.

Despite all that, the Celtics still had a chance to make a late push and simply couldn’t muster any sort of sustained charge.

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Here’s the sequence that stood out to us and it’s one that highlighted that second-chance struggles: Boston had clawed within five with under five minutes to play when Marcus Smart stonewalled a Pascal Siakam attempt to back him down. Siakam kicked the ball out and Kyle Lowry attacked from the perimeter but Daniel Theis shuffled over to force a miss.

The ball ticked off the front rim with both Theis and Smart in position to snag the rebound. Instead, Lowry’s hustle helped the ball bounce free inside the charge circle. Smart stumbled a bit but he and Theis were still in position to potentially grab the ball as it bounced towards the end line. Instead, Boston’s duo watched it land out of bounds with the referee immediately signaling that Toronto maintained possession.

Lowry, who likewise motioned emphatically in the direction of the Raptors' basket, took his position as inbounder and lobbed a pass over Jayson Tatum that caused Boston’s All-Star to turn his head. With that, Lowry was gone, sprinting to a spot on the left wing where he took a return feed from Game 3-hero OG Anunoby, watched Tatum get snagged by a double screen from big men Siakam and Marc Gasol, and drilled another 3-pointer.

That's a microcosm of the series. The ball was there for the taking and Boston just let it bounce away. The Raptors, who had already dominated the hustle stats for the first 43 minutes, pounced again. And, no surprise, it was Lowry at the forefront.

The fourth-quarter (lack of) rebound wasn’t an isolated incident. Just look at Boston’s general indifference to this rebound chance just 85 seconds into Game 4. Lowry runs past three white jerseys to snag the offensive rebound. It was one of the rare times that Toronto didn’t immediately score off the second-chance opportunity.

Boston’s response to its Game 3 loss wasn’t good enough. Players openly admitted that they got outworked by the Raptors. That simply cannot happen after the way the last game ended.

“They played harder than we did and I think that was just noticeable on both ends of the floor for most part of the game,” said Tatum.

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Echoed Kemba Walker: "They had a great intensity throughout the whole game. I thought we just didn’t match those guys’ intensity. But it’s the playoffs. We’ve just got to be better. That’s it. We’ve got to be better.”

In the ramp to Game 4, we noted how Boston’s response would tell us a lot about their championship potential. Game 3 produced the first real adversity that the Celtics have seen in months, and they needed to respond with a championship resolve.

It didn’t happen. Instead, it was the defending champs that again showed their mettle. Boston didn’t play with any sort of desperation. The Celtics missed shots, they made careless turnovers, and they never really made the sort of surge that truly threatened the Raptors.

“At the end of the day, we just have to be better," said Walker. "We have to find ways, especially down the stretch. There were times we got stops, but we didn’t finish the play out. Those guys just out-toughed us and got loose balls, got offensive rebounds, things that we weren’t doing. That’s really it. We’ve got to be better. We’ve got to be tougher. We’ve got to want it more.”

OG Anunoby’s 3-pointer at the end of Game 3 might have breathed life into a series that was a half-second away from being over. If the Raptors lose that game, there’s a good chance Boston sweeps them out of the bubble on Saturday.

Now the Celtics are the ones pondering how things unfold from here. Can they muster a better response in Game 5? They better find some intensity fast. Toronto has all the momentum in the series.

Boston, for much of the first three games, looked like they were clearly the better team. Not on Saturday. Lowry was a monster. Even at age 34, he logged 44 minutes of floor time. His shot wasn’t perfect but he made 4 of 10 3-pointers and finished with 22 points, 11 rebounds, 7 assists, 2 steals, and 2 blocks. Yes, he had five turnovers, but no one on Boston came even CLOSE to matching his intensity. Lowry was undeniably the most impactful player on the court.

All this while Walker mustered only nine shot attempts. Jaylen Brown, who twice dubbed the Game 3 loss a “f—ing disgrace,” missed 14 of the 18 shots he put up. He tried changing his shoes at halftime and even the new ones didn’t have any buckets.

Celtics coach Brad Stevens marveled that his team was even in the game based on their shooting woes. After pleading with his team to simply focus on the next play, he watched his players hang their heads after missed shots. Said Stevens: "We weren’t very crisp all night. Once we missed a few, we started trying to hit home runs.”

Stevens leaned on Robert Williams early then seemed reluctant to go back to him. This despite the fact that Williams has changed the energy on the floor during many of his stints. Some of this is on the player, who has to maintain his hustle and avoid the defensive lapses. But Stevens might also have to simply live with the miscues given how important Williams could be to both this series and anything Boston accomplishes beyond this.

The Celtics will almost certainly shoot better in Game 5. After bottling up Siakam for much of this series, they allowed the Raptors All-Star forward to get going a bit and didn’t offer quite as much resistance in the paint.

Ultimately, this series might hinge on who wants it more. On Saturday, it was clearly the Raptors. Boston has been able to survive poor shooting nights because it’s hustle and defensive tenacity can keep it in games.

Boston can’t get outworked and expect to beat a team like Toronto. The Celtics need to play with a different level of desperation on Monday.

Said Brown: "We have to be ready to fight for our lives next game.”