Brandon Bass laid on the ground clutching his knee, his teammates gathered around him looking like their child just got kicked in the face, Doc Rivers walked over and shook his head, and for a moment the Boston Celtics’ fragility was never more clear, like the tightly-wound ball of yarn building for the last two months had been yanked at the end and it was ready to all come unraveling.

“Well, I thought he was hurt,” Rivers said. “I’ve had that injury — we know the one I’m talking about? I don’t even like saying the word.”

“I just told him to get up,” Rajon Rondo explained.

But Bass “felt like a little kid. I was scared.”

The problem with being a veteran team without interior depth of any kind is that all it takes is one awkward fall and everything Rivers dreams of, every resilient win and how-are-they-still-this-youthful run and they-shouldn’t-be-this-good-anymore moment can give way to hopelessness. This Boston Celtics team has been surging for some time now, pulling together at the right time, emerging from near-.500 at the All-Star break to round into a unit that now looks every bit the part of title contender. They are the most resilient team Rivers has ever coached, by his accord. They have developed an identity as the little tank engine that could, with superstars that should be over the hill, key role players who never accomplished anything before this season, and an attitude that no matter what happens, they will continue charging forward.

But Bass went down and everything was threatened. The ball of yarn was either unraveling or it was being scorched in flames or Justin Verlander had it in his right arm ready to throw it halfway across the world.

“Scary moment,” Kevin Garnett described.

But the Celtics can breathe. Bass hyperextended his knee but he didn’t even have to leave the game. He stood up gingerly, limped around a little bit and stayed in the lineup to help close out Boston’s latest win. Afterward the Celtics could even joke about the moment, could joke about the desperate few seconds when everything they work for was jeopardized, when the team’s mortality never felt more alive.

“The guys were laughing that he was exhausted and he needed some rest,” Rivers noted.

Bass added, “They said a bunch of things. Some said I was tired. Some said I was acting, I’d gone Hollywood. But I was scared and it was hurting, too. I didn’t want to let the team down.”

He didn’t let the team down. Instead, he provided the most breath-seizing reminder that these Celtics are one important injury from being irrelevant in the playoffs, from having Ryan Hollins as their lone backup big for the season’s stretch run. All these thrilling wins, these victories the entire Celtics fan base should be proud of, the ones that weren’t supposed to happen this season, not like this, they all mean something by themselves but could mean a lot less in the larger picture because one injury could push the Celtics straight to life support.

This run has been beautiful and unexpected and validating, but in one moment, it could end before it’s supposed to.