Joe Mazzulla was named the Celtics’ interim coach just days before the season and now, as the permanent coach, will lead Boston into the playoffs starting Saturday. Steven Senne/Associated Press

When the Boston Celtics suspended Ime Udoka on the eve of training camp, Brad Stevens trusted Joe Mazzulla enough to make him the team’s interim head coach despite Mazzulla’s inexperience and an organization going through uncertainty.

In February, that confidence continued to carry through as Stevens took the interim tag off Mazzulla and gave him a contract extension.

Now comes the true test of Stevens’ faith in Mazzulla, who will coach his first career playoff game when the Celtics open the first round at home on Saturday against either the Heat or Hawks.

Mazzulla guided a successful regular season for the Celtics, who finished with 57 wins and the No. 2 seed. But the playoffs – and coaching a team with championship-or-bust aspirations – presents a different, more complicated challenge.

Stevens’ confidence isn’t wavering based on the circumstances, though. Asked Sunday if Stevens, the Celtics’ former head coach, will communicate with Mazzulla more as the playoffs begin, he pushed back.

“We talk every day, but I want to go back to this: Joe is a strength,” Stevens said. “He’s done a really good job. I understand because he’s new that the easiest thing to do is nitpick him, but he’s done a really good job. If he needs me, I’m here, but I trust him and I trust the staff, and they’ve all done a good job. I think our players would all second that.”

At this time last year, Mazzulla was a second-row assistant coach with the Celtics. The 34-year-old’s only prior head-coaching experience came at Division 2 Fairmont State for two seasons. But there was no hesitation from Stevens to name him Boston’s head coach at a tumultuous time.

The day after Udoka was officially suspended, Stevens raved about Mazzulla’s sharpness and talent as a basketball mind, his leadership qualities and ability to galvanize a room.

So far, it seems Mazzulla has rewarded Stevens’ faith. Stevens showed that with the extension in February, and he’s been satisfied with what he’s seen after Mazzulla’s first regular season.

“He was gonna be really good,” Stevens said. “That’s been pretty obvious for a long time. But he’s been consistent in his own approach, win, lose, he comes back and works the next day. He wants to grow, wants to improve. He demands that of the team. I think he does a good job of picking what the emphasis needs to be in the big picture and also in those small moments, those snippets of games where something is waning a little. He does a good job of figuring that out and making that a priority to improve.”

Mazzulla’s first regular season certainly didn’t go without hiccups. His timeout usage – or lack thereof – was always a talking point. In some blowout wins, he kept his starters on the floor longer than they probably should have. Mazzulla recognized his imperfections and mistakes, which were naturally going to happen for a first-time NBA head coach.

On the floor, Mazzulla said he knew he wanted to make the Celtics both a top offensive and defensive team because that’s been the makeup of recent champions. He hammered home the details and preached the margins as keys to victory. But his humility may be his greatest strength.

He took a step back, recognizing his inexperience and that he didn’t know everything. He prioritized collaboration and relied on his coaching staff and an experienced roster that’s been through so much together. He rarely got in the way, and while he certainly hasn’t been perfect, it was a recipe for success.

“You just don’t know what you don’t know,” Mazzulla said. “I think you have to go through certain stuff in order to be aware of it, in my opinion. There’s just some stuff that you can’t simulate. And so, I think my focus was on, what’s the most important thing at this time? And then along the way you see some of the stuff you need to get better at. You handle each situation different. And so, I think I’ve just tried to be open-minded and flexible throughout the entire year, understanding that it’s not going to go well all the time. At the same time, I have a lot of good people around me and we’re going to figure it out. And so, I think if you can just maintain that open-mind perspective, it will help you.”

Mazzulla has passed most of his tests to date. He guided the Celtics through uncertainty at the start of the season to the league’s best start. When they hit a lull after the All-Star break, Stevens was happy with how Mazzulla’s consistency helped them get over it. Now they’re hitting their stride going into the postseason after a strong finish to the regular season.

Now comes the ultimate test. Stevens believes Mazzulla has gotten better as the season has progressed, that he’s navigated the season as well as possible given the circumstances. Will that carry over to the most important games of the season? Time will tell.

“If you watch in the last couple of months, he’s calling the game as he sees it now even more so than before,” Stevens said. “So he’s quicker to stop a run, he’s quicker to do things. He may stop a game so he can make a sub. He may let a game go so they can’t make a sub. There’s a lot of decisions to be made in that moment and you kind of feel out how your team best needs you and that takes time. That takes time for a person that’s done it for 30 years and that takes time for a person that’s never done it before and he’s done a good job.

“I think he’s just done a good job. And again, I think it goes to … the process. All he wants to do is learn and grow and he’s great with that. I think that he’s been a great strength for us.”