Celtics center Al Horford defends against Nets forward Kevin Durant during the Celtics’ 115-114 win in Game 1 of their first-round playoff series on Sunday in Boston. Steven Senne/Associated Press

When the Celtics acquired Al Horford from the Oklahoma City Thunder last summer, no one was quite sure how much the former All-Star had left in the tank.

Boston made the deal partially for financial reasons, easing their salary burden from Kemba Walker’s max deal to Horford’s more reasonable cap hit of $27.5 million. However, team sources insisted to MassLive at the time of the deal that personnel was just as big of a factor with the acquisition. Brad Stevens knew Horford fit well with Jayson Tatum, Jaylen Brown and Marcus Smart and finding more of those players was crucial to get this team back on an upward trajectory.

Boston initially declined to give Horford the type of commitment he got from Philadelphia in the summer of 2019 because of concerns about how his game would age. However, as a 35-year-old in Game 1 of the first-round series against the Nets, he pushed that skepticism to the background as he has all year long.

Tatum and Smart got most of the shine after Boston’s thrilling 115-114 win but the team would not have had a chance for the comeback victory without Horford’s stellar play. He posted 20 points and 15 rebounds in the victory, a playoff career high for him in Boston while outplaying the Brooklyn bigs for the 41 minutes he was on the court.

“Al was Al, he does it all the time,” Celtics Coach Ime Udoka said. “I know it doesn’t always show up on the stat sheet, but playing 41 1/2 minutes, I know we have a three-day break coming up catching up on the rest, but he’s doing everything we’re asking him for. Guarding the bigs, playing in touch coverage, to switching, had some great contests on Durant late, and then we knew he had an advantage on the glass, especially when they go to their small lineups and we’re switching, we think we can take advantage of that. Really encouraged him and Theis to go after the offensive boards and that was huge. The third to the last basket he had a putback. He was great obviously.”

With Daniel Theis and Grant Williams struggling with their shots (4 for 13 from the floor, 0 for 6 from 3-point range), having at least one big to keep the Nets defense honest from the perimeter was crucial. Horford’s efficiency (8 of 13 FG) stood out and helped keep the Nets’ top rebounding weapon Andre Drummond off the floor in crunch time given his limitations defending the 3-point line.

Horford was also integral to the team’s comeback in the final minute of the game. He showed a hard double against Kyrie Irving on Boston’s final defensive possession, refusing to let the Nets point guard turn the corner against him as Irving nearly drove across the entire width of the floor with his right hand.

“The play before that I think he hit an extremely difficult shot over Marcus,” Horford said of defending Irving. “You couldn’t play better defense than that. At that point, we needed to do something different so we went ahead and did that. Just try to get a stop. We talked about getting a stop in that moment and we were fortunate enough to finish with the rebound, too.”

Ultimately, Irving was forced to dish out the ball to Kevin Durant with the shot clock winding down, opening the door for Horford to secure prime rebounding position against a smaller Nets lineup. The 35-year-old quickly got an outlet pass to Derrick White, putting the Nets in scramble mode as the Celtics pushed the ball in transition for the final 12 seconds of the game, ultimately leading to Tatum’s go-ahead layup.

“It’s a learning experience for our group,” Horford said of the final sequence. “I know that we’ve all had playoff experience in the past but it’s the first time that this group is together, first playoff game against an extremely good team. The biggest thing is the composure. We continued to stay with it and that’s what we did, stayed with it and took their shot and were able to stay with it and find a way to finish the game.”

The Celtics are still a long way from winning the best-of-seven series but the team’s odds of survival without Robert Williams look just a little bit better now with Horford playing at this level. The 15-year veteran is performing like he’s in the prime of his career and that makes Boston’s second half turnaround look like it could translate to a deep postseason run.