In 2009, the Boston Celtics and Chicago Bulls met in the first round of the NBA playoffs for an incredible series that set the record for most overtime games (four).

The defending champion and 2 seed Celtics, who hadn’t lost a playoff series since trading for veteran stars Kevin Garnett and Ray Allen, eventually prevailed after seven tenuous games.

Despite the loss, the future looked incredibly bright for that Chicago team. After all, they were led by rookie of the year Derrick Rose and seemed to be poised to contend in the Eastern Conference for the next decade.

We all know what happened next: Rose tore his ACL in the 2012 playoffs, hasn’t been able to stay healthy since, and was traded to the New York Knicks this past offseason.

Fast forward to today, and these teams seem to have completely switched roles. The Celtics have the third-youngest roster by average age in the NBA at 25.6 years, and led by star guard Isaiah Thomas, finished with the best record in the East.

On the other end of the spectrum, the Bulls confused many by making their big offseason splash signing veteran guards Dwyane Wade (35 years old) and Rajon Rondo (31 years old) to hefty offseason contracts worth $47 and $27.4 million, respectively. The Bulls finished a tumultuous season at 41-41, just good enough to sneak into the playoffs via the tiebreaker over the Miami Heat, drawing the Celtics in the first round.

Given only a 34.75% chance to win the series according to our algorithms, the Bulls came into Boston and took Game 1 by a score of 106-102.

After taking game one and home court advantage back the Bulls are now the favorites to win the series with a 55.63% chance to advance according to our numbers.

So what happened in Game 1 that enabled the Bulls to beat Boston? Can Chicago sustain this level of play and upset the 1 seed in the Eastern Conference, a feat which has only been done three times since 2000? Let’s get into it.

Game 1 Recap

Before getting into the actual numbers, the series started in emotional fashion after the tragic loss of Isaiah Thomas’ sister, Chyna, less than 24 hours before the game. Despite the heavy heart, Thomas still managed to put together a very solid game, scoring a game-high 33 points, which he backed up with 6 assists and 5 rebounds as well.

But, scoring wasn’t the issue for the Celtics in game one -- it was rebounding. The Celtics got killed on the boards by Chicago, losing the rebound battle by 17 total and by 8 on the offensive glass.

This isn’t a new problem for the Boston. During the regular season, Boston ranked a lowly 25th in rebounds, pulling in a mere 42 per game. Chicago was near tops of the league, ranking 3rd with 46.3, a full 4 rebounds ahead of Boston. In terms of rebounding rate, the Celtics ranked 27th at 48.5%, and the Bulls were 4th at 51.7%.

Boston was also unable to hold Jimmy Butler in check, a hallmark of their regular season victories over Chicago where they held the star guard to a mere 36.2% shooting.

In Game 1, Butler tidily put together his most efficient game against the Celtics so far, going for 30 points on only 19 shots from the field as well as sinking 3 of his 5 tries from three-point land. Butler also attacked Boston’s interior defense and drew fouls, going to the free throw line 12 times and making 9 of them.

The final nail in the coffin against the Celtics was the bench performance of Chicago. The Bulls were plus-13 in the bench scoring department, and more than half of their 35 bench points were scored by rookie Bobby Portis. In perhaps his best game of the season, Portis shot 80% from the field and 75% from three, scoring 19 points, nearly as many points as the entire Celtics bench managed to score.

Keys to the Rest of the Series

The Big Man Battle

Al Horford was Boston’s marquee signing this offseason, as general manager Danny Ainge handed the former Atlanta Hawks big man a four-year max contract worth more than $113 million. However, Horford didn’t quite live up to his expectations.

According to our nERD metric, Horford (3.6) was a positive player, adding 3.6 expected wins to an average team based on his efficiency, but that ranked him 61st in the NBA and was well off the mark of Thomas' 16.1.

But this isn’t even the most concerning part of Horford’s season. The center’s total rebound percentage declined for the fifth consecutive year, down to 11.8%, far from the career high of 18.0% he set in 2008. This lack of rebounding was evident in Sunday’s game, as Robin Lopez dominated Horford on the boards.

Lopez grabbed 8 of his 11 rebounds on the offensive glass, constantly outmuscling Horford and giving Chicago second chances to score points.

The Charity Stripe

Boston and Chicago were two of the top free throw shooting teams in the league this past season, finishing third and sixth in team free throw percentage, respectively. In Sunday’s game, Chicago displayed their prominence at the line, sinking 20 of 23 attempts, the most important of which were Butler’s two with seven seconds left, giving the Bulls a four-point lead.

The Celtics only managed to shoot 73%, but far more worrying than this number is that, other than Thomas, only three other Celtics shot free throws. Other than Amir Johnson, (who led the way with three) no other Boston player shot more than two. This simply has to change if the Celtics look to beat the odds and win the series.

Takeaways

Homecourt advantage was already key to this series, and with the Bulls already taking this back with their Game 1 victory, Thomas and the Celtics simply have to win Game 2 at home or risk falling into an insurmountable hole.

An interesting note to end on: the last of the three 1 seeds to lose to an 8 seed? The Chicago Bulls, in 2012 to the Philadelphia 76ers.

According to numberFire Live, the Celtics are 67% likely to even the series with a Game 2 victory.