If any Celtics players believed Joe Mazzulla would mellow out now that he’s an NBA champion, they quickly discovered that was not the case.

If anything, Boston’s eccentrically intense head coach is pushing his team even harder as it begins its bid for back-to-back titles.

Multiple Celtics veterans said training camp, which began Wednesday in preparation for the club’s preseason trip to Abu Dhabi next week, has been unusually challenging thus far.

“Training camp has been hard,” Jaylen Brown said after Friday’s practice. “Training camp has been a lot of conditioning, a lot of defensive stuff, setting the tone on the defensive end, pushing ourselves. It’s been great. It’s exactly what we needed.

“We did not ease into training camp by no means. Joe Mazzulla is a psycho — in a good way. Training camp was not — it was probably one of the tougher training camps, the first two days, that I’ve probably had in terms of intensity conditioning, level (of) physicality, defensive, warrior-type mentality.”

Guard Jrue Holiday, who’s played for six different head coaches in his 15-year NBA career, offered a similar review.

“(It’s been) hard,” Holiday said Thursday. “Tough days. I got traded (to Boston) last year and I missed the first day (of camp). I wish I missed the first day of this year, too. But hard days. Obviously, we want to come back this year and do it again. This is where it starts. We got to set the tone early the first week. … There are things that we want to set now, set foundations and we’ve been doing that the first two days.”

Day 1 of camp featured “a lot of running,” according to backup guard Payton Pritchard, and mental preparation was a point of emphasis during the entire opening week. Sam Hauser told The Athletic’s Jay King that Mazzulla placed bubbles over the hoops during one drill, forcing the players to focus on everything but shooting.

“I think it’s very, very mental,” Holiday said. “Obviously, that’s where most of the game can be won. Concentration, doing things while you’re tired, playing without passing, playing without scoring. How do you win the mental game, I feel like, is the biggest part – if you’re tired physically or mentally. Again, he’s just throwing everything at us.”

Mazzulla knows this breakneck practice pace might not be enjoyable for his players, but he’s been pleased with their buy-in thus far. The Celtics returned nearly their entire roster this offseason, so the vast majority of their current players are familiar with Mazzulla’s methods.

“I’m probably much happier to see them than they are to see me,” Mazzulla said. “Which I’m OK with. What I love is just that these guys set the tone. They’re the ones that are here first in the building. They set the environment, and I enjoy watching guys go after greatness. You get here early; there’s guys working on their individual game. … So I enjoy just the environment that they create, them and the staff.”

Brown said the early exertion is what the team needed after a celebratory summer.

“I think it (has been) good for us,” the NBA Finals MVP said. “Now, we’re just working on building great habits, breaking habits from just being off in the summer and working on trying to play great Celtic basketball.”

The Celtics are scheduled to practice once more at the Auerbach Center on Sunday before flying to the United Arab Emirates, where they’ll square off against the Denver Nuggets in exhibition games Oct. 4 and 6. They’ll then close out their preseason with two games at TD Garden and one in Toronto before opening the regular season at home against the New York Knicks on Oct. 22.

The last six NBA champions all failed to repeat the following season – with the last five bowing out before the conference finals – and Mazzulla wants to ensure the Celtics aren’t skipping any steps as they attempt to end that drought.

“It’s not about getting off to a fast start. It’s getting off to the right start,” the coach said. “Making sure we’re doing the things we’re supposed to be doing. Are we in the right mindset of competing, of executing, working to get better? How are our practice habits? How are our game habits? So it’s not about getting off to a fast start. We’ve just got to make sure we’re in the right mindset and we’re executing.

“Through three days, we’ve done that, and we just have to continue to try to make that. Regardless of what the record is, it’s more about where we’re at mentally and how we’re approaching practices, games, stuff like that.”