BOSTON — On opening night, when their prized free-agent addition went down for the season in agony, the Celtics badly needed an emotional and physical boost.

The Celtics knew they still had stars in Kyrie Irving and Al Horford and young, rising playmakers in Jaylen Brown and Jayson Tatum. These four players have already stepped up and become a huge factor in the six-game winning streak that followed an 0-2 mark to open the year.

But with six months of the regular season and a hopeful playoff run ahead, there was a distinct sense the team would need a pick-me-up, an occasional smile and gregarious personality to take command and remind everyone things would be OK without Gordon Hayward.

Enter Aron Baynes, the 6-foot-10, 260-pound thunder from Down Under. The 30-year-old veteran has an irrepressible spirit that is spreading throughout the Celtics locker room. Brad Stevens has turned to Baynes to provide some valuable leadership on the court, on the bench, and in the locker room.

“Definitely a tough situation. Not anything that any of us were expecting or wanting,” Baynes told me. “Now trying to regroup, and Brad has been great at that. He’s really focused us in, and he’s trying to get the guys in the right position. That’s what he’s great at. When we play within his system and do the right things, then we find ourselves in good circumstances. Different guys have had more opportunities and they’re making the most of that, which is great to see. We’re just trying to cover up as best we can. You definitely can’t replace what we lost in Gordon. But we’re trying to make the most of it. Our system really covers for what we don’t have out there.”

Stevens described the defensive intangibles Baynes, along with the 31-year-old Horford, bring to the low-post defense of the Celtics this season.

“You have to build a trust. You have to be loud, you have to communicate and you have to care about that end of the floor every minute of every day,” Stevens added. “We really have a group that's committed to that. They've built a good trust together. I think we have good directors on the back line of our defense with our bigs, guys that can call out coverages and kind of point people where to go. When you're talking about Baynes and Al, those guys have been through a lot of different things and seen a lot of different things, and they can see the whole floor in front of them.”

Through eight games, the Celtics are ranked fifth in the NBA in field goal percentage allowed at 36 percent, are the only team to hold Giannis Antetokounmpo to under 60 percent shooting (and they’ve done it twice), and rank sixth in rebounding.

Whether he’s joking with fellow Australian (born) Kyrie Irving about homegrown roots or suggesting a great Italian eatery 35 miles from Boston in Worcester, Baynes is the guy keeping everyone loose.

There were plenty of pieces coming and going this past offseason, but in the midst of Danny Ainge’s additions of Hayward, Irving and rookies Jayson Tatum, Semi Ojeleye and Daniel Theis, the most significant piece in terms of chemistry and toughness wears No. 46.

But Ainge didn’t bring in Baynes to be a comic or a big brother to the Celtics. He was brought in to be a bruiser and a rebounder, dropping anchor in the paint on offense and defense.

Baynes is that enforcer down low who takes some of the burden off Horford and does the dirty work in the paint. The Celtics have had gritty players of late, just not the size to match. No disrespect to Jae Crowder, Tyler Zeller or Kelly Olynyk, but that wasn’t their role.

Far beyond the career averages (5.3 points, 4.1 rebounds and 14.2 minutes per game), Baynes brings that intangible attitude the Celtics have lacked, that physical answer to Tristan Thompson and LeBron James down low. It wasn’t so much the rebounding numbers as the way the game was being played close to the basket that Ainge needed to address. When hitting free agency this summer as a 30-year-old veteran, Baynes needed a team, and the Celtics needed a true, old-fashioned big.

“Well it was a great situation,” said Baynes, who signed for one year and $4.3 million. “I’ve seen what Brad has done with the team and how he’s been able to take them on long runs in the postseason. I wanted to be in a situation where I could come in and contribute and help us get back to that same type of circumstance. It’s been fun being here and just trying to make the most of the opportunity, which I’m thankful for.”

With Hayward out, Marcus Morris slow to come back from a balky knee, and Stevens wanting to free up Horford to play to his strengths at the 4, it’s been Baynes who has stepped into the starting lineup and given the team the ability to keep everyone else in their natural positions—that’s a big, big deal.

When Gregg Popovich made the comparison to Dave Cowens, there were more than a few eyebrows raised. Even Baynes wasn’t all that familiar with the Celtics legend. But the Spurs coach knew exactly what he was talking about—he coached Baynes for Baynes’s first three NBA seasons. The Spurs coach said that both players hustle, go all out on every single play, and leave everything out on the court.

“I think I also heard him say that I’m not in the same caliber of that,” Baynes said with a smile. “But it’s always good. I appreciate what Pop has to say about me. It just goes to show that he still cares. Which is good. I really appreciated my time in San Antonio and learned a lot from him. Still now, he puts out some pretty good messages for us. You try and read between the lines and what I take away is that he likes what I’m doing and I’m going to keep doing the same thing to try and help my team.”

In his first season in San Antonio, Baynes backed up a future Hall of Famer in Tim Duncan and won an NBA title when the Spurs took down LeBron and the Heat in 2014.

“He was the first guy I ever looked up to when I started playing basketball and I got to win a championship alongside him,” Baynes said. “Throughout that I saw what he does and how consistent he is and what he brings to the team and just to the court every single time. His routine allows him to be so consistent. At the same time, he was the biggest dog on the court, but he had time for us young pups out there and have a chat with us and really just show us and lead us in the right direction. He was more of a quiet leader than anything. You see that guy going out there and doing it and think, well if he is going out and doing it, there is no excuse for me and you go out and try to emulate it.”

Now, with Hayward slowly working his way back into the team’s routine following surgery, Baynes will have another player in his ear, offering bits of advice.

“He’s a great basketball mind and a great bloke off the court as well,” Baynes said of Hayward. “Getting the chance to have him around here, and I’m not sure what role he’ll have, but he might be watching a bit more video and being able to instruct. Coach Hayward, Why not? He’s already a student of the game. We’re definitely excited to have him back around the team and whatever input he has, every one is going to listen. He’s shown over the last few years how good he is and how well he sees the game. So it’ll be fun to have him around.”

The journey for Baynes has already been a remarkable one. From his high school roots in Mareeba and Cairns in Queensland, Australia, to Washington State, to Europe, and then back to the NBA, it’s a lot for even Baynes to imagine. But now, this year with the Celtics, there could be a few more memorable moments in store.

“Hopefully there are still a few more things to go through right now,” Baynes said. “I’ve enjoyed it and it’s gotten me to here. But I’m definitely not satisfied. I’m still trying to take a few more steps. I’m enjoying it right now and it’s a great situation that we’re in. So I’m thankful to be here, around these great guys that we have in this locker room.”