By Matt Citak

As the famous sports saying goes, “it ain’t over till it’s over.”

But heading into Monday’s Game 1 between the Boston Celtics and Philadelphia 76ers, it sure seemed like Boston’s chances of winning were as close to zero as you’re going to get prior to tip-off.

The Celtics entered the first game of their Eastern Conference Semifinals matchup without arguably their three best players. Gordon Hayward was lost for the season in Boston’s first game of the year, while Kyrie Irving was ruled out for the remainder of the season a month ago. If that weren’t enough, Jaylen Brown, who stepped up in a big way during the opening-round series against the Milwaukee Bucks, suffered a hamstring strain in Game 7 and was forced to miss Monday’s game.

Even without their big three, along with backup big man Daniel Theis, the Celtics managed to put a beatdown on the 76ers, taking the opening game of the series, 117-101.

Philadelphia has been perhaps the hottest team in basketball over the last month and a half. Heading into this series, the 76ers had lost just one game since March 13, going 20-1 during that stretch.

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But the undermanned Celtics lived up to the “next man up” mantra, proving it to be more than just a classic sports cliche.

Terry Rozier, filling in for the injured Irving at point guard, poured in 29 points, including 7-for-9 from the three-point line, eight rebounds and six assists. Rozier was absolutely amazing, combining his great shooting ability with uncanny ball control.

But Rozier was not the only Celtic to go off Monday night. Jayson Tatum added 28 points, shooting an impressive 91.7 percent from the free throw line on 12 attempts. In addition to Tatum, veteran big man Al Horford shot 10-of-12 from the field, finishing the game with 26 points and seven rebounds.

With each of their outstanding performances, the Celtics witnessed a trio of players go for more than 25 points in a playoff victory for the first time since 1986.

The trio that accomplished the feat 32 years ago? You might have heard of them. Their names are Larry Bird, Kevin McHale, and Dennis Johnson…

This feat is even more impressive when you consider the all-time trio of Celtics that won a championship 10 years ago. That’s right, a backup point guard that was playing in the G-League two years ago, a rookie, and Horford combined for more points than Kevin Garnett, Paul Pierce, and Ray Allen (three Hall of Famers) ever combined for during their time with the Celtics.

That is simply impossible to fathom.

But that’s how Brad Stevens has this Celtics team playing at the moment. No matter who is on the court, these Celtics are going to challenge every shot on defense, fight for every loose ball, and continue to battle until the clock says 0:00.

Now this was only Game 1 of a best-of-seven series, and the 76ers are still favored to advance to the next round. And in my personal opinion, I believe Philadelphia will bounce back and end up taking the series.

But if Monday showed us anything, it’s that we cannot sleep on the Celtics.

Irving and Hayward will remain out, but Brown can be expected to return at some point in this series. Even if he misses one more game, the worst-case scenario is he comes back with the series tied 1-1. And if Boston can put up an overall performance like they did Monday without Brown playing, who knows what they can do when the talented sophomore returns to the court?

As the great Terrell Owens once said, “get your popcorn ready,” because this is going to be one wildly entertaining series.

Matt Citak is a contributor for CBS Local Sports and a proud Vanderbilt alum. Follow him on Twitter.

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