Marcus Smart, left, Jayson Tatum, center, and Kemba Walker were key to the Celtics success this year and will be moving forward. The question is what does Boston need to add to get past the Eastern Conference finals. Mark J. Terrill/Associated Press

The Boston Celtics’ season ended Sunday with a loss. The Celtics make the playoffs most seasons. When you make the playoffs and you don’t win a championship, which is most seasons, the last game is a loss.

Falling 125-113 to the Miami Heat is not an indictment against Boston in a broader sense, except in that falling behind 2-0 and 3-1 is not a viable way to win a series, despite the precedent set by the Denver Nuggets. Three Eastern Conference finals in four years is an impressive accomplishment, and only this season could the Celtics have realistically expected to make the finals. In 2017, a hobbled Isaiah Thomas and the Celtics were trucked by LeBron James. In 2018, even getting to the Eastern Conference finals was noteworthy with Kyrie Irving and Gordon Hayward injured. The top two scorers, Jayson Tatum and Jaylen Brown, were a rookie and second-year player, respectively. The future was too young and too bright for real pessimism, or even criticism.

But now, the dynamic is a little different. The Celtics are a little older, a little better. They could have beaten Miami and had a prime opportunity to reach the finals. They are contenders now, and fair or not, they can’t really afford to wait any more. The clock is ticking.

Tatum will likely sign his rookie extension, because players generally sign rookie max extensions. What then? Will he be good enough to lead the Celtics to the title? It looks that way, and he will be expected to do so. If he is, the Celtics need to advance past the conference finals. If they don’t, what moves will the Celtics have to make over the next few years to keep him happy?

Brown has worked fanatically and improved exponentially in Boston. What is his ceiling? We haven’t seen it yet. How will the Celtics maximize him?

Kemba Walker will turn 31 in May. I am 31. I can tell you with some certainty that you are not completely washed at that age. But now, you are at the peak staring over the edge, which is a very different feeling. Time might not be as kind to you going forward, you might think if you were 31.

Anyway, Walker’s knee might be a legitimate problem, which is troubling.

“Anything I tell you would be an excuse,” Walker said after the game. “But I was fine. I was able to get on that court, and that’s all that matters.”

Hayward has now suffered traumatic injuries to both ankles. He returned to the bubble, finished rehab, missed the birth of his first son and hobbled through three games.

After doing literally everything in his power to try to help the Celtics win, he now faces the uncomfortable task of opting in to the final year of a player option – an uncommon result from a season like Hayward’s.

These are questions the Celtics will need to think about. They might not resolve themselves, but they could. Tatum’s next steps might make him into a superstar whose ascent can’t be denied. Brown might take another leap. Walker might have years left on his knee. Hayward might rally with months to rehab his ankle.

The Celtics also have a lot of options. They could try to swing a big trade for a disgruntled star – a constant reality in the NBA. That’s exciting, but risky. As the Miami Heat have proven with Jimmy Butler, a formerly disgruntled star in the right situation can be a perfect setup (and perhaps Butler in Boston would have been a better pairing of personality and city than some would like to admit). As the Philadelphia 76ers have proven with Butler (and Tobias Harris and Al Horford), however, packaging assets for the wrong player can undo years of careful reconstruction in a single offseason.

The Celtics could also do some work around the edges. The center position feels most likely to be filled internally by some combination of Daniel Theis, Robert Williams and Grant Williams, but Boston might have been tougher this year with additional scoring punch off the bench alongside Smart and whichever starter is being staggered. Davis Bertans may have been a pipe dream, but other shooters are out there.

The draft looms large as well. The Celtics have three picks, which can be either used or maybe packaged to move up. R.J. Hampton and Tyrese Maxey can both score. Moving up to target Devin Vassell could be productive. We will write much more about the draft in the coming weeks.

Those are all good scenarios. In them, the Celtics have a talented young core and plenty of potential for the future. Those things were true both before and after they blew leads against the Heat and finally bowed out with a cold shooting stretch at the end of Game 6.

The clock is ticking, though. The Celtics are contenders now, and contenders have expectations. They have a great collection of young talent, but that talent is ready to win, which requires a win-now mindset from an organization.

Take it from me and Brad Wanamaker, who turned 31 in the bubble as well: You are young until you aren’t, at which point you just need to be an adult taking care of adult things.

That moment comes sooner than you think.

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