By now the Celtics were losing by 35 points with 12 minutes of garbage time ahead. But their fans would not relent.

“Let’s go Celtics,” they caroled again. They were like the inhabitants of Whoville on Christmas morning, and LeBron had become their Grinch. His Cavaliers had ransacked their home – the Celtics had never led in any of the three games here while losing them by a combined 90 points – and yet they didn’t appear to care about any of that.

“Our offensive mistakes led to bad defense, and it just kind of snowballed on us,” said coach Brad Stevens after his young Celtics had ended their extraordinary season with a 135-102 loss Thursday in Game 5 of the Eastern Finals. “It’s a good step forward as a season as a whole, but I leave with a little bit of a taste in your mouth … you want to play well here because there is really no place like TD Garden in Boston. The fans at the end of the game—man, it’s just amazing.”

Their fans were neither naive nor ignorant so much as they were appreciative of how NBA championship teams are built. It is a hard, slow process. James, arguably the most talented player ever, had needed nine seasons and a change of uniform to win his first title. The Warriors, who will be opposing Cleveland for a third straight NBA Finals, had invested six years in developing a championship roster around Steph Curry.

The Celtics, in year four of their renewal, had reached the conference finals before using the upcoming No 1 pick, before spending their cap space this summer, and before cashing in another potential high pick that may yet arrive next year via their 2013 swindling of the Nets.

“We’ve got a good amount of people that will be back, and obviously, a strong core with also some exciting opportunities in the draft,” said Stevens. “It’s pretty cool to think that in [four] weeks you’ve got the No 1 pick in the draft. It means that I’ve got to go straight to work tomorrow, but I’m looking forward to watching some of these prospects.”

The environment of Game 5 was a strange swirl of the recent past and the long-term ahead. In this era of unrealistic demands and kneejerk criticism, the fans of Boston – America’s winningest sports city in the new millennium – were expressing foresight as well as appreciation for all that this team had accomplished. Consider that the top four of the Cavaliers –James (35 points on 18 shots to go with eight assists and eight rebounds), Kyrie Irving (24 points and seven assists), Kevin Love (15 points and 11 rebounds) and Tristan Thompson – had all been Top 5 draft picks. They were complemented in the rotation by Deron Williams (a former No 3 pick) and Channing Frye (No 8).

The Celtics, by comparison, had yet to acquire so much natural talent. Apart from their trio of Top 10 picks – Al Horford (No 3), rookie Jaylen Brown (No 3) and Marcus Smart (No 6) – the roster was defined more accurately by its overachieving surprises. Thomas, their All-Star, and small forward Jae Crowder were former second-rounders who had come to Boston with little expectation. Crucial postseason roles had been played by Avery Bradley, Terry Rozier, Gerald Green and Jonas Jerebko – all having arrived outside the lottery.

And yet they had finished No 1 in the East. They had won the first round after mourning the death of Isaiah Thomas’s sister, they had won a Game 7 against the Wizards in spite of the hip injury that would end Thomas’s season, and they had won Game 3 in Cleveland (and threatened to steal Game 4) in his absence. They had won the lottery without tanking, and their young players had developed while winning 53 games.

For other franchises, the NBA’s Final Four is a destination. For these Celtics, at this particular place and time, this postseason run was prologue.

There is likely to be little complaining about the blowout losses at home – not with so much to look forward to this summer. The newspapers and talk shows in Salt Lake City have already begun to ask whether the Celtics (to be led by Stevens, his former coach at Butler) will recruit Jazz free agent All-Star Gordon Hayward. Markelle Fultz, Lonzo Ball or Josh Jackson may arrive as the No 1 pick – if it isn’t traded for someone better. Celtics president Danny Ainge may also try to package several of his emerging prospects for an established star.

The Cavaliers proved in the opening minutes of Game 5 that the Celtics must upgrade their talent in a meaningful way. Love scored Cleveland’s first eight points in a span of 53 seconds. A couple of minutes later James was blocking a layup and then stealing an entry pass for successive transition layups. Altogether the defending champions were getting back in transition and anticipating Boston’s ball movement.

“The start of the game was so bad, I felt like it was hard for us to come back,” said Bradley (a team-high 23 points) after watching the Cavs shoot 64 percent in the opening quarter while building a 21-point lead. “Even when we scored six or seven points in a row, we were still down by 20 points.”

When Bradley turned a couple of Cleveland mistakes into breakaway dunks, his fans stood in applause as if their Celtics weren’t trailing 73-57 at the intermission. Then the second half opened with Crowder crowding Irving before turning around to see his fall-away 3-pointer extend Cleveland’s lead to 21 points. “Kyrie just goes nuts and ends us,” said Stevens. “Those were really tough shots.”

With 2:44 remaining, the home team losing 125-93 and the arena less than half full, the chorus began again. “Let’s go Celtics,” they sang for the remainder of the game, and as the players left the court their fans gathered around the exit to applaud the season in totality and the promise of better years to come. This lopsided defeat had provided them with reality, and from where they stood that reality was not so bad. The NBA Draft was 28 days away, and it was not so bad at all.

By Ian Thomsen