In the end, a thin bench proved costly for the Celtics — and Danny Ainge deserves blame for that

The Heat's Bam Adebayo was too much to handle for undersized Celtics center Daniel Theis.Mark J. Terrill/Associated Press

Sign up for Court Sense, our Celtics newsletter

ORLANDO — Thirteen years ago, Celtics president of basketball operations Danny Ainge knew he had a championship contending team when he acquired Ray Allen through trade and convinced Kevin Garnett to accept a move to Boston from Minnesota. He then packed his roster with veterans with championship pedigrees, special skills, and leadership capabilities.

A basketball generation later, Ainge built another team with those same championship aspirations, using the draft and free agency to build a strong core with enough talent to make a long playoff run.

But when it counted, Ainge did little to enhance his roster to compete against the league’s elite, and it cost the Celtics a chance at a title.

Danny Ainge couldn't get the Celtics to a championship this season.John Tlumacki/Globe Staff

A team that was flawed at the trade deadline stayed flawed because Ainge made no moves, and the team’s stark weaknesses were exposed during their Eastern Conference final series against the Miami Heat, one the Celtics could have potentially won with a better bench.

While the Heat added Jae Crowder and Andre Iguodala and the Lakers added Markieff Morris, Ainge let his roster ride it out until it ran out of gas. The Celtics desperately needed outside shooting from the bench, and there were available players who could have helped, but Ainge stayed committed to players who weren’t going to help.

Vincent Poirier signed a two-year deal to help at center, but he was rarely on the active list during the playoffs. Javonte Green was a popular player on the roster, but he was never going to break into the guard rotation.

Now Ainge has a chance to work on this roster and upgrade it so it can reach its potential. The issue is the likely shrinking salary cap and the salaries of Jaylen Brown, Kemba Walker, and Gordon Hayward that will limit flexibility. But the Celtics need to upgrade the center position.

Is Daniel Theis the long-term answer in the middle for Boston?Mark J. Terrill/Associated Press

Daniel Theis enjoyed a solid season as a first-year starter but his lack of offensive prowess, getting to the free throw line and inability to contain bigger centers defensively cost the Celtics in the Miami series. The Heat’s Bam Adebayo scored in a variety of ways and in Game 6, he even brought the ball up the floor and attacked Theis off the dribble to score baskets.

Fourth-quarter scoring is difficult when you don’t have players who can get easy buckets. Without a big man who can get to the free throw line — Theis averaged fewer than two attempts per game in the playoffs — it puts more pressure on the perimeter players. The Celtics have played without a real offensive force at center for years, but defensively they were able to make a difference. Theis, unfortunately for Boston, has proven unable to handle big, agile centers. And in this case, it pretty much cost them the Miami series.

If Ainge is into reunions, Aron Baynes is a free agent and the Celtics could bring him back on their midlevel exception. Ainge could also use draft picks to perhaps facilitate a deal for a frontline center, but they are difficult to find. The organization has to decide what to do with second-year center Robert Williams, who they drafted as their version of Adebayo in a sense but Brad Stevens didn’t trust him enough to give him quality minutes in the Miami series.

What does the future hold for Robert Williams?John Tlumacki/Globe Staff

It’s difficult to see such an athletic specimen sitting on the bench when the Celtics are getting gashed in the paint. But while Williams has the instincts to become a rim protector, his pick-and-roll defense and other basketball acumen was not always present.

Lack of bench strength also left a lot to be desired as the Celtics advanced deeper into the playoffs. The bench had been an issue all season for the lack of production. And with Marcus Smart starting because of Gordon Hayward’s ankle injury, a hobbled Hayward became the most viable bench option.

Brad Wanamaker had his playoff moments as a solid backup point guard but Stevens was relegated to using undersized Grant Williams at center in stretches, along with Enes Kanter, who struggles on defense. This is where Ainge’s lack of activity at the trade deadline was costly. While the Heat brought Tyler Herro off the bench — and he won Game 4 with his 37-point outing — the Celtics had no one close as a counter.

Hayward was playing at about 65 percent, according to an NBA source, and lacked lift on his jump shot and eventually lost confidence. The Celtics desperately need bench strength and experience. They need a quality shooter who can stretch the floor and who is capable of an occasional breakout game.

What sort of impact could a healthy Gordon Hayward have on the roster?Mark J. Terrill/Associated Press

A healthy Hayward will return to the starting lineup and Smart will become part of the bench unit again but the reserves had been an issue all season. There were no wily veterans or “old heads” to pull Tatum or Brown aside for guidance. Ainge obviously didn’t want to disturb his current draft assets so he passed on deals but that was a costly mistake.

Ainge needs to use the Memphis pick (14th overall) to take the best shooter in the draft, a player who can help next season. The club thought it may have that in last year’s second-round pick Carsen Edwards, but he barely played as a rookie and the club will have to determine whether him, Tremont Waters, and Tacko Fall are part of their future.

The roster was cluttered with too many players who just couldn’t help this team this year. The biggest miss was Poirier who was expected to give the team center depth but eventually became a developmental player. So it’s time for Ainge to take a page from his previous GM guide and build a roster that’s ready to compete for a championship, not just a starting five.