But these Celtics are used to using doubt to fuel them. It's been that way since Gordon Hayward was injured on opening night of the regular season in Cleveland. And few expected the Celtics to stick around long in the postseason, even after winning 55 games and claiming the No. 2 seed in the East.
"You've got everybody counting us out, but we knew from the beginning we've got each other," Celtics guard Terry Rozier said after Boston's practice on Friday, its first since dispatching the Philadelphia 76ers in five games.
"We weren't gonna lay down for nobody, no matter who was counting us out. We weren't going to just let the teams come in and punk us. We were the No. 2 seed for a reason. With injuries, without, we were the No. 2 seed, so obviously we were doing something great all season and what [a reporter] said is true, you can keep counting us out.
"We'd rather [the media] count us out. We don't want anybody on our side now. We're doing good and we play better when our back is against the wall."
The Celtics were underdogs for the first four games of their second-round series against the Philadelphia 76ers, and only a minor favorite despite playing Game 5 at home earlier this week.
Celtics coach Brad Stevens said before the playoffs that he loves seeing "narratives busted." And he knows that his players have fed off the perception that they can't be competitive while playing without injured All-Stars Gordon Hayward and Kyrie Irving.
"We talked about it prior to the playoffs, nobody should love a challenge more, nobody should have more fun doing it," said Stevens. "I think our guys have done a good job doing that."
Of course, the Celtics understand full well why they are underdogs in this series, particularly with the way James is playing and the relative ease with which these Cavaliers swept through top-seeded Toronto in Round 2.
Boston players know they have a tough challenge ahead in trying to slow down James.
"[James' postseason numbers don't] surprise me at all, man. It's just, physically, he is far more superior than anybody that's on the floor," said Celtics swingman Jaylen Brown, who will be tasked with defending James as much as anyone. "[He is] 250-plus pounds, can run like a gazelle -- physically he's just unstoppable. But we just gotta be mentally locked in and have the mindset to come in and try to do it the best we can."
"With this group, we have more depth in [the perimeter] positions, suited more to guard LeBron," said Horford, who was part of last year's team that lost to the Cavaliers in five games. "We have a lot more bodies to take on that challenge."
Brown noted that James is going to get his points, but the Celtics can't let him take over games.
"Our goal is not to stop LeBron, it's to win the game," said Brown. "That's our mindset."
Added Rozier: "We understand he's gonna make some crazy plays. That's LeBron. But we've got to try and make it as tough as we can. He's not gonna make every crazy play every time down. We've just got to show bodies and show our energy and length."