The speed with which Jayson Tatum recovered from Achilles surgery didn’t surprise the Celtics.

Head coach Joe Mazzulla said Friday, before Boston’s All-NBA wing took the court for the first time since last May, that the team always expected Tatum to return this season.

Why? Because Tatum was fortunate enough to have his Achilles repaired less than 24 hours after he ruptured it during a second-round playoff loss at Madison Square Garden, and because Mazzulla witnessed firsthand how dedicated he was throughout his rehab.

Tatum sat out the Celtics’ first 62 games before making his season debut Friday night against the Dallas Mavericks at TD Garden. His recovery process lasted 298 days — just shy of 10 months.

“Really since Day 1, it’s just been a constant conversation,” Mazzulla said pregame. “Throughout that process, we’ve just trusted his journey. And I give him credit for — we always knew he was coming back this year. I think I knew that when he decided to have surgery within a 16-hour span or whatever the case was. But you give him credit for the vulnerability and the communication throughout the journey. There’s always steps along the journey that are going well, and there’s always days that are not going well.

“So I think being open and vulnerable about the journey has been good for him, and I think the other piece about just being around the team every day was good for us as a team and also good for us as a player, and again the support staff piece of what (Celtics trainer) Nick Sang has done, what our sports science team has done, his family has done. I think just constant communication, listening and being part of the journey. And here we are.”

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Mazzulla later amended his answer to clarify that the team knew Tatum would be healthy enough to return during the season, leaving open the possibility that he might not have if the Celtics fell out of contention without him.

But Boston maintained its status as one of the NBA’s top teams while its best player was sidelined, entering Friday’s game with the league’s fourth-best record and second-best net rating. It now gets to add a player who finished in the top six in NBA MVP voting in each of the last five seasons to a deep, hard-playing roster with championship aspirations.

“Because of the decision he made right away (to have surgery) and because of the way he attacked it, and really because of the people around him, I knew he was going to do everything he can to get to a decision to where (he felt) like I can do this,” Mazzulla said. “Like I said, it’s a credit to him and to the people around him.”

Getting Tatum back with 20 regular-season games remaining, Mazzulla said, is “not much different than acquiring a new player in the middle of the season, to where you have to quickly build a relationship and earn that, and apply that to a game the next night.”

“I’m excited about the challenge and the opportunity that we have, which is what we have all the time, right?” the coach said. “I think at the end of the day, our team is whole now. I mean, that’s just the reality. There’s different ways to lead, there’s different ways to be a part of the team, there’s different ways to impact the team. So we’re at the point now where we have a guy back, and with that comes the opportunity for us to do different things, to add to what we as a team up to this point have done.

“So there’s multiple things there, right? It’s like, hey, we have a group of guys that have done a great job up until this point, and now we make an addition, and now we have a chance to be even better. And how do we do that as a team? And so anytime you have opportunities to be able to do that, I think it’s fun. And that’s what I’m looking forward to.”

Mazzulla would not share details about how he planned to work Tatum back in — asked whether he’d be on a minutes restriction, the coach replied: “I don’t know” — but he did confirm he would start against Dallas. Second-year pro Baylor Scheierman shifted to the bench to make room for the returning superstar.

“It’s been a long journey, and the culmination of a new journey begins today,” Mazzulla said. “It’s a credit to him for the work ethic that he’s put in, and the support staff — his family and also the support staff of the Celtics — getting to this point. So now we start another one.”