Like many people, I’ve spent this season doubting the Boston Celtics.

After watching Gordon Hayward suffer the worst injury I’ve ever seen in person, I wrote “it’s hard to see the Celtics as anything but a mid-tier playoff team in the Eastern Conference, at best.” After Kyrie Irving was lost for the playoffs after multiple knee surgeries, I predicted the Milwaukee Bucks would knock them out in the first round in six games, thinking they wouldn’t have enough offense to win the series. And after surviving that series, I predicted the Philadelphia 76ers would knock them out in five games, thinking Joel Embiid and Ben Simmons would prove too dynamic for the injury-riddled Celtics to overcome.

Now, of course, the Celtics have reached the Eastern Conference finals, where they’ll face the Cleveland Cavaliers in a repeat of last year’s matchup.

Clearly, prognosticating Boston’s chances this year has been some of my best work. Or not.

I still stand by all of those predictions – I was far from alone in making them. But that doesn’t change how wrong I was.

It would be one thing to just own the loss and move on. But I’m a believer in looking where I went wrong, in an effort to avoid making the same mistakes. So here is a list of the things that led to the Celtics surpassing so many people’s expectations – mine included – en route to the Eastern Conference finals:

1. Al Horford

Many people would expect to see Celtics Coach Brad Stevens atop the list. But Horford has been a constant, stabilizing force for a young team, especially through the first two rounds of the playoffs.

In his nine previous postseason appearances, he scored 20 or more points 13 times – and 25 or more just twice – in 92 games. In 12 playoff games this season, Horford has already scored more than 20 points four times, including matching his playoff career high of 26 points twice.

His average of 17 points per game these playoffs is easily the highest of his career.

More importantly, Horford has anchored Boston’s defense. As a rare versatile big, Horford was versatile enough to defend Embiid in the post, sharing the responsibility with Aron Baynes, who allowed Stevens to single-cover Embiid, and Simmons on the perimeter, helping cause the presumptive rookie of the year’s series-long struggles.

Just look at this stretch from Horford in the final minutes of Game 5, when the score was 94-94 with 6:41 to go:

n Horford scores in the post to make it 96-94 with 6:31 left.

n Horford makes a jumper to make it 98-94 with 5:53 left.

n Horford steals the ball from Dario Saric, leading to a Terry Rozier basket to make it 100-94 with 5:27 left.

n Horford assists Jayson Tatum on a dunk to make it 102-96 with 4:57 left.

That’s four straight baskets created by Horford. He finished an alley-oop with 1:27 remaining before stealing the ball from Saric again to set up Tatum’s winning basket with 22.5 seconds left.

Horford always has done a little of everything, and done it all very well. He’s a perfect piece for Stevens to build around and, as the lone healthy all-star on the roster, has been the rock around which the coach has constructed this roster.

2. Boston’s youth playing beyond its years

Some of this is obvious, right? Jayson Tatum would be rookie of the year in almost any season that wasn’t this one – Simmons and Donovan Mitchell had two of the best rookie seasons in recent memory – and Jaylen Brown took a significant step forward in Year 2.

But as Boston hit the playoffs, its young players, not only Tatum, looked like they had playoff experience. On the other hand, the Sixers’ four best players – Embiid, Simmons, Dario Saric and Robert Covington – had never played in the postseason.

Yes, Boston is full of young talent. But Brown played in 17 playoff games last year. Terry Rozier played in 22 playoff games the previous two years, and Marcus Smart had played in 28 in the previous three. That might not seem like a lot, but it’s hard to look at what the Celtics did over the past week and not think that their young players were far better off having gone through this experience for the first time. Think Embiid, Simmons, Saric and Covington won’t be better in the postseason next year?

That’s no explanation for Tatum, who looks wise beyond his 19 years every time he’s on the court. But the Celtics, as a whole, are a young, experienced team. The Sixers, as a whole, were a young, inexperienced team. That, in the end, is why Boston advanced.

3. Brad Stevens

Stevens has done a remarkable job since team president Danny Ainge plucked him from Butler University five years ago. The things that always stood out about Stevens at Butler – preparing his teams, getting a wide-open shot or layup out of every after-timeout situation, developing players – has carried over in spades in Boston.

The Celtics started, at a minimum, four first-round picks in the playoffs, including three No. 3 overall picks (Horford, Brown, Tatum). It’s not as if Stevens hasn’t had any talent, and the media narrative that Stevens is a magician who is conjuring this success out of whole cloth is a slight exaggeration.

But Stevens has obviously done a remarkable job. And his greatest feat is his ability to put his players in an optimal position to succeed.

Stevens has created a system in which Rozier can step in for Irving or Tatum, and Brown can step in for Hayward. Players are empowered to fill a role that fits their skillset, and given the confidence they can do so.

The credit for the creation of that system, and the development of the players within it, goes to the coach.

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