Before Sunday, Drew Peterson had played a total of 30 NBA minutes in his professional career.

Not 30 games. Thirty minutes. Such is life for a two-way player on the league’s deepest roster.

But with the Celtics recently hit by a rash of injuries, illness and other absences, Peterson received a sudden and unexpected promotion this week.

The 6-foot-9 wing played 25 minutes Sunday night in a narrow road loss to the Cleveland Cavaliers while Boston was missing Jaylen Brown and Derrick White. Then, he played another 23 on Sunday as the Celtics routed the Miami Heat 108-89 without Kristaps Porzingis, Jrue Holiday, Al Horford and Sam Hauser.

Head coach Joe Mazzulla did not alert Peterson — who’s averaging 19.6 points and 7.0 rebounds per game in the G League this season — ahead of his abrupt spike in playing time. But he came away encouraged by the 25-year-old’s performance.

“We played against two good defenses, two teams that really protect the paint, and two teams that have length and physicality,” Mazzulla said after Monday’s win. “And so to me, he’s done a great job. One, he plays with a level of confidence. Two, I always like guys who are better defenders than people think. You said that with Sam and Drew — he’s a better defender than people think he is. He can think the game in real time, and so he can understand what teams are trying to do on both ends of the floor, and he can adjust to that really quickly, and he can make shots.”

Over the two games, Peterson shot 4-for-11 from the floor and 4-for-10 from 3-point range, made 3 of 4 free throws, scored 15 points and registered one assist, one steal and 11 rebounds. He grabbed a pair of offensive boards against Miami (the team that originally signed him as an undrafted free agent last year) and hit two threes.

After his second make, Peterson stared down his former teammates on the Heat bench.

“(Monday night), everyone will look at the 2-for-5 (shooting), but he had seven rebounds, and so he has the ability to impact the game in different ways,” Mazzulla said. “He has the ability to think in real time, and he can defend.”

After the Cavs game, Mazzulla said the Celtics “try to find moments throughout the season to where you can validate the work that the guys put in, regardless of who it is.” Players are expected to stay ready and be prepared for those fleeting moments when they do come.

Peterson wasn’t the only member of that so-called Stay Ready Group to see minutes Monday. Down two starters and two of his top three bench players, Mazzulla also added Jordan Walsh, Xavier Tillman and first-round rookie Baylor Scheierman to his rotation, with Scheierman seeing the first meaningful playing time of his young career.

All of those players finished with positive plus/minus ratings against a Heat team that also was depleted by injuries.

“I thought it was great,” Brown said. “It’s a journey, man. We need each and every guy. We need the whole chain to be working together. So, shoutout to those guys being ready, and shout out to Joe for giving them those opportunities throughout the year to be able to showcase and add to winning. It takes a complete group. And I thought those guys stepped in.

“I thought Drew was great. I thought even Baylor came in, made plays. And each and every night, you never know when your name is going to get called. You’ve just got to be ready. And those guys have been professionals, and that’s what we need going forward.”

Happy for Peterson

Before joining the Celtics last December, Peterson spent time with the Heat’s G League affiliate in Sioux Falls. Heat head coach Erik Spoelstra said he was happy to see the former undrafted free agent getting minutes — even if it had to be with one of Miami’s chief rivals.

“We liked him, too,” Spoelstra said pregame. “We make this agreement with the guys: We want to make it happen for them one way or another. They have big dreams. We want to be dream-makers. We wish we could do everything with us, but that’s just not the reality. We want to develop guys as much as possible, and it ends up being sometimes a situation where it has to be somewhere else. We only have three two-ways; we can’t call everybody up.

“But we’re thrilled when they do get an opportunity, even if it’s somewhere else — even if it happens to be Boston. Anywhere else, we’d be probably happier. But we want to make it happen one way or another. We can get out of our way on that one, too — maybe.”