As the Celtics prepare to jet off to Abu Dhabi for a pair of preseason matchups with the Denver Nuggets, here are nine takeaways from the opening week of training camp for the reigning NBA champions:

1. This isn’t an easy camp

Jrue Holiday missed the start of Celtics training camp last year after his late-September trade to Boston. “I wish I missed the first day of this year, too,” the veteran guard joked.

Boston’s camp practices have featured Joe Mazzulla’s trademark intensity, with the head coach immediately testing his players both physically and mentally.

Holiday said Mazzulla has been “throwing everything” at the team.

“Tough days,” he said after the second day of practice. “… Hard days.”

Jaylen Brown summed it up nicely: “Joe Mazzulla is a psycho — in a good way.”

Mazzulla acknowledged he’s “probably much happier to see (the players) than they are to see me” but said he’s been pleased with their buy-in thus far.

“What I love is just that these guys set the tone,” he said. “They’re the ones that are here first in the building. They set the environment, and I enjoy watching guys go after greatness.”

2. Mazzulla loved Tatum’s Olympic DNPs

Jayson Tatum said his reduced role on Team USA probably made his head coach “the happiest person in the world.” Mazzulla confirmed that, saying Tatum was “100% right.”

“I don’t think we’ve seen the best of him yet because of how much he works and how he’s willing to grow,” Mazzulla said. “So I thought it was great that he has something to work toward.”

3. Tatum is taking steps to fix his shot

After struggling as a shooter during both the NBA playoffs and the Olympics, Tatum has been working with trainer Drew Hanlen to clean up “a few mechanical things” in his jumper.

“There’s some things I could’ve fixed, but just in the midst of the playoffs trying to manage your rest and things like that, it’s a little tougher,” said Tatum, who still led the Celtics in points, rebounds and assists during the postseason.

“I was still playing well, just wasn’t shooting the ball as well as I would have liked, and obviously we were winning so it wasn’t the time or place to try to fix things in that moment.”

Like Mazzulla, Tatum believes his best basketball still is ahead of him.

“You haven’t seen the best version of Jayson Tatum,” the 26-year-old said.

Boston Celtics big man Kristaps Porzingis speaks during Media Day at the Auerbach Center on Tuesday. (Photo By Matt Stone/Boston Herald)

4. Porzingis is progressing well

President of basketball operations Brad Stevens said the Celtics are “very, very pleased” and “maybe a little surprised” with where Kristaps Porzingis is in his recovery from offseason leg surgery.

Porzingis is targeting a December return to the court but hasn’t ruled out an earlier comeback, saying he hopes to resume playing “as soon as possible.” He’s already running and taking light contact.

Odds are Boston’s starting center will miss at least the first month of the regular season, though, and the Celtics will lean on a combination of Al Horford, Luke Kornet, Xavier Tillman and Neemias Queta to replace him.

5. Two reserves had offseason surgeries

Kornet (wrist) and Tillman (knee scope) both had offseason procedures done to fix lingering injuries. Those were far less intensive than Porzingis’s and both were full-go in time for camp.

Tillman said his knee would swell up after games last season, which contributed to him missing 28 contests. He’s played in 60-plus games just once in his career and hopes to be healthy enough to hit that mark this season.

6. Baylor Scheierman bulked up

At the Celtics’ urging, their first-round draft pick put on 10 to 15 pounds of muscle in the two months between Summer League and the start of camp to help prepare for the physicality of the NBA.

Scheierman, a prolific offensive player in college, knows he’ll need to prove he can defend against professionals to have any hope of cracking the Celtics’ rotation.

“Offensively, I feel very confident in myself being able to impact the game on the floor,” the 6-foot-6 Creighton product said. “I know that in order for me to get minutes on the floor, it’s all going to come down the defense. So that’s where I’m trying to hang my hat on and just trying to get better every single day in that category.”

Mazzulla said Scheierman “had some great physical defensive plays” in his second practice, showing improvement from Day 1.

Scheierman also has found a veteran mentor in teammate Sam Hauser, who’s been helping the 24-year-old rookie adjust.

“He’s a good shooter, but he’s a lot more than a shooter,” Hauser said. “He’s got a good feel for the game. … I think he has a good future in this league.”

Boston Celtics Jaylen Brown walks into the press conference room during Celtics Media Day at the Auerbach Center. (Photo By Matt Stone/Boston Herald)

7. Jaylen Brown hit the gym

Speaking of bulking up, Brown arrived at camp noticeably buffer than he was in previous seasons. He said he added muscle to ready himself for what he hopes will be another long playoff run.

Brown also opened up about how the Celtics’ long-awaited championship helped him mentally, leading to what he called “an awesome summer.”

“I mean we got close five, six seasons we were in the conference finals, so I had six heartbreaks before I got to finally feel what it’s like to win,” he said. “So this summer, my heart wasn’t broken.”

8. Lonnie Walker IV is taking the right approach

A player with Walker’s NBA resume (25 years old, 322 career games, 88 starts) really should be on a roster somewhere rather than having to prove himself through an Exhibit 10 tryout. But that’s his reality after failing to land any guaranteed deals in free agency, and the veteran wing is trying to make the most of his opportunity.

“I’ve got to earn it,” Walker said. “Obviously, there’s a lot of different aspects that might carry onto it, but being on an E-10, it’s not guaranteed. I’m not coming here with the mindset of, ‘OK, I’m on an E-10 and afterwards, I’m going to get the guaranteed contract.’ It’s like, I’ve got to work my tail off. I really have to, each and every day, put it all out on the court. First one in the gym, hopefully the last one out, come back later on, shoot and just be mentally and physically prepared for the outcome.”

Walker made 38.4% of his 3-pointers for Brooklyn last season, averaging a career-high 20.1 points per 36 minutes. Teammates and Mazzulla have praised him for his professionalism.

9. The Celtics aren’t satisfied with one ring

The quote of camp, from Tatum on media day: “It was never just about trying to just win one. Now you get to at least be in the same room with the other Celtics great teams, great players. All the guys I looked up to growing up won at least one championship. Now it’s just a conversation of, how great are you trying to be? What room or what tier are you trying to be mentioned in when it’s all said and done? And understanding the window you have to maximize that time.”