Blaming the Celtics' 129-120 overtime loss to the Brooklyn Nets on injuries would be easy.
Key guys for Boston were either out (Jayson Tatum) or limited (Kemba Walker) coming into the game Tuesday night, while others suffered injuries (Gordon Hayward, knee bruise and Jaylen Brown, hamstring) that kept them off the floor in the closing moments.
Still, that's not why they suffered what will surely be remembered as the worst loss of the season and arguably one of the worst of the Brad Stevens Era.
In-depth news coverage of the Greater Boston Area.
For one of the rare times this season, the Celtics looked like a team that became comfortable with being comfortable.
And that includes Stevens, whose lack of adjustments late in the game was among the many factors contributing to Boston (41-19) losing its second game in a row.
Stevens acknowledged after the game that he could have done a better job of managing the game.
"I should've subbed a couple of guys out that normally play," Stevens said. "And I just let them… kept them in, and we had great intensity for most of the game and we just let our foot off the gas."
And the end result was Caris LeVert absolutely smashing the Celtics' defense to the tune of 51 points - yes, 51 points.
The Celtics didn't play with the kind of edginess and purpose we are accustomed to seeing from this group, which more than the loss, was surprising.
Celtics fans never have to worry about the players morphing into human bobbleheads.
Or so we thought.
This team has been living a charmed life most of this season and now has hit its first real stretch of adversity.
It isn't the fact that they have lost back-to-back games at home.
It isn't all the injuries, either.
The bigger concern is that the Celtics aren't playing with the kind of mental toughness needed to win what are clearly winnable games.
This time of the season, that's a huge issue when you start talking about playoff seedings, postseason matchups and all the little things that could mean the difference between a first-round knockout or a deep postseason run into May and early June - the primary goal for the Celtics this season.
And the frustration of the recent stretch seems to be wearing on some, notably Marcus Smart, who had to be restrained from the officials for the second game in a row.
Indeed, Smart's fiery temper has been a contained blaze most of this season, only to erupt in the past two losses.
If he does play against the Cavs on Wednesday (he will likely be fined for his eruption on Tuesday, or the NBA may suspend him for a game), will he ratchet the emotions down and play with more control, or will he snap like we've seen the past two games and make inexplicable plays like the foul he committed with 0.2 seconds to play against Brooklyn that led to LeVert's tying free throws?
Boston goes into Cleveland with a ton of questions, most of which can't be answered until they step on the floor with whoever is healthy enough to play.
And by healthy we're talking both mentally and physically ready to compete, which, for the first time in a long time, is far from a given when it comes to this team.
Don't miss NBC Sports Boston's coverage of Celtics-Cavaliers, which begins Wednesday at 6:30 p.m. with Celtics Pregame Live, followed by tip-off at 7 p.m. You can also stream on the MyTeams App.