BOSTON — Boom.

It sounded like a sledgehammer was hitting behind Al Horford's locker stall inside TD Garden and Boston’s veteran big man can feel the reverberations as he puts on his shoes. But Horford just smiles and shakes his head.

“I have a lot of faith in those concrete walls,” said Horford.

Boom.

These loud thuds became a familiar white noise in the Celtics locker room after games this season. But, by late April, it felt like the noise was getting louder. Marcus Morris’ stall is a safe distance from the epicenter but even he couldn't help but notice the decibel level.

“Shem, god damn,” said Morris.

Boom.

Further across the room, Terry Rozier heard the noise but always figured it was just part of the perpetual construction happening around the Garden. Gordon Hayward, who is as far from the pounding as possible, was more aware of what was happening in the adjoining weight room.

"That dude is a machine, man,” said Hayward.

Boom.

Eventually, the rhythmic pounding subsided. Then it started up again. It was audible as Celtics players answered questions about that night’s game. A media relations staffer would sometimes stand near the door between the locker room and weight room, aiming to muffle the noise by pulling the door closed whenever players and coaches passed through.

Boom.

Finally, after the locker room had all but cleared out, the door swung open and a sweat-covered Semi Ojeleye, still in his game shorts and tank top with headphones over his head, emerged.

Those booms were a medicine ball that Ojeleye had repeatedly launched into a wall as part of a CrossFit-like routine that Celtics rehab and performance coordinator Zach Markowitz had designed for Ojeleye on nights he doesn’t get into game action.

It used to be that Ojeleye would only engage in these postgame workouts on the road, yearning for some way to burn off pent up energy and avoid the drudgery of sitting in a hotel room. But as the “Did Not Play — Coach’s Decisions” piled up on a Boston roster loaded with talent this season, Ojeleye decided to add a postgame workout to the mix before departing TD Garden.

"Zach just kinda puts this stuff together. It’s kinda like some CrossFit-type stuff, so we do some rotations with med balls and kettlebell swings and bike work,” said Ojeleye. "It’s a lot of different stuff but trying to get some dynamic work in because you’re not getting that game action.”

Jaylen Brown hears the nightly thumping and finds himself concerned only by the force he figures is needed to make the locker room shake the way it does when Ojeleye starts his medicine-ball routine.

“I don’t know what his deal is, why he’s so angry,” Brown said while trying to muffle a sly smile. “I get it, he’s probably like, ‘Man, f— it, I wanted to be out there [in the game].’ He’s just throwing that medicine ball and he’s probably got a picture of Brad Stevens next to him.”

At this point, Brown starts laughing and repeatedly declares he’s just kidding. No, he marvels at the way Ojeleye has handled his situation.

"Semi is a true professional. And nothing short of it,” said Brown. "He handles every situation the right way. When he gets in and when he jacks himself up, he’s ready, mentally engaged. He is a true professional. I’ve learned a lot from Semi, you know what I mean? And I’ve been in the league longer than him and I’ve learned from Semi, just waiting on his situation, his level-headedness, his matureness, his work. He’s a great asset to this team and the organization.”

READY AND ABLE

No one who’s ever gotten a glimpse of what Celtics Twitter dubbed Ojeleye's “thick, jacked frame” has ever doubted his dedication to the weight room. But his relentless desire to stay ready despite never knowing when his next opportunity will come has endeared Ojeleye to just about everyone in the organization.

“He’s definitely the hardest working guy on this team,” said Hayward, who often shoots before and after practice with Ojeleye and is no stranger to his weight room exploits.

Echoed Horford: “That’s the kind of guy you want on your team. If I could have Semi Ojeleye on my team for 15 years, I’d be very happy, just because he’s so professional and he works extremely hard.”

Ojeleye logged a mere 594 minutes in 56 appearances this season. That’s nearly half of his total floor time from his rookie season, when injuries increased his opportunities.

Ojeleye played a mere 28 seconds in Boston’s first-round series against the Indiana Pacers. But that’s about to change. Celtics coach Brad Stevens has routinely called on Ojeleye in matchups against the Milwaukee Bucks, tasking the brick-wall big man with the impossible task of making potential league MVP Giannis Antetokounmpo work for his points. And Stevens has full confidence in Ojeleye in that situation.

"Semi’s always the most reliable guy ever,” said Stevens. "You just know exactly what you’re getting every day with his work ethic.”

WHAT'S IN A NAME?

Jesusemilore Talodabijesu Ojeleye’s page on Basketball Reference lists six nicknames , starting with the familiar Semi (pronounced Shemmy), then adding, Muscles Jesus, The Ox, Thor, The Ojeleye Factory, The Man Made of Granite. Needless to say, the 24-year-old wing is well regarded for his muscular frame. Teammates tell tall tales of his workouts while assistant coaches lament the amount of time they’ve spent just trying to find him a gym on the road.

But in a league where young players with DNPs are often the first to sneak out the door and into the night, Ojeleye’s dedication to putting in postgame work is the reason all of his teammates think he’s going to eventually be a star in this league.

"I think it’s the hardest thing in sports to do, to mentally try to maintain some sort of focus and discipline and still be a professional, not knowing if you are going to get 2 minutes, 5 minutes, no minutes, 15 minutes,” said Hayward. "I think for most guys, you kinda know when you’re going to go in the game, how many minutes you’re going to get. Sometimes, in some systems, you know the shots that you’re going to get each game, different things like that and you can really prepare.

"But in [Ojeleye’s] situation, you just don’t know. And that’s tough. Like I said, it’s very impressive, the professionalism he has at such a young age.”

There’s a line of thought that, if the Celtics weren't so well stocked at the wing position, with an ability to mix and match with the likes of Hayward, Jaylen Brown, Jayson Tatum, and Marcus Morris, that Ojeleye would be a more consistent contributor. He almost certainly will be a rotation player in future seasons. But the fact that he doesn’t sweat his current uncertainty and simply stays ready for the next opportunity is why teammates gush about him and revel when he does get into games.

"As much as he would want to play more, I’m sure, he understands his role and he’s always prepared,” said Horford. "That’s difficult to do. It’s always nice to see him be rewarded with getting to play.”

For his part, Ojeleye admits it can be tough playing the waiting game, but he’s unfailingly positive about his situation.

"I just try to reframe that as it’s a blessing to be here,” said Ojeleye, who is more likely to post scripture to his Instagram story than any of the blinged-out rock star-like photos that young NBA players tend to spotlight. "There are some days when it’s tough, you’re like, ‘Man, I wish I could get my rhythm and know when it was going to happen.’ At the same time I think you have faith that it’s going to happen, then you do what you can by putting in the work and that kinda makes it easier, it helps me focus on the positives.”

KEEPING IT 100

Ojeleye laughs when asked to put a number on the total number of medicine ball throws in a typical postgame session. “Let’s say 100,” he offered. Reporters like to joke that he must be working out with a vision of defending Antetokounmpo in mind, but Ojeleye chuckles at that suggestion as well.

“I’m just listening to music and really it’s -- I feel like, during the game, I didn’t get that work in. I didn’t help my team. So I’m trying to catch back up. Just trying to prepare for the next opportunity.”

This summer, as part of TD Garden’s ongoing renovations, the Celtics' tiny outdated locker room is scheduled to expand. That will include a relocation of the team’s weight room. So it won’t be an issue if, as some teammates expect, Ojeleye ends up launching a medicine ball straight through the dividing wall this season.

Teammates very much enjoy those booms that echo through the locker room.

"The beast, he’s preparing,” said Horford. "He’s waiting to be unleashed.”

And Horford isn’t tipping the game plan when he notes that the unleashing is upon us. The Celtics have enough talent now that it would seem more likely that Hayward or Morris would elevate to a starting role in Boston’s small-ball lineups but it’s fair to expect a heavy dose of Ojeleye and Aron Baynes as the primary big-man defenders off the bench.

“I think this series, [Ojeleye will] probably get an opportunity here,” said Horford. "His ability to space the floor and make an open shot, but then also to defend. And he defends Giannis, and defends at a high level -- he’s a great weapon that we have.”

THE BUCK STOPS HERE?

One of the more common Ojeleye nicknames that you won’t find on Basketball Reference is the “Giannis Stopper.” It’s slightly exaggerated because, well, no one has figured out how to stop Antetokounmpo, particularly during this 2018-19 campaign when he’s made an emphatic case as the best two-way player in the NBA.

And, yet, it’s undeniable that Stevens deployed Ojeleye in a starting role in Games 5-7 of last year’s playoff series against the Bucks. He trusts Ojeleye to make things difficult, though player and coach would be among the first scoff at the suggestion of any sort of “Giannis Stopper.” Heck, “Giannis Slower” might be too much.

The Celtics have typically fared well when Ojeleye is on the court with Antetokounmpo.

The pair logged 41 minutes together on the court during the Celtics-Bucks regular-season matchups this year. Antetokounmpo’s net rating with Ojeleye on the floor was minus-4.5, then spiked to plus-7.4 in the 66 minutes he played without Ojeleye on the court. Now, that’s a bit misleading because the Bucks had an offensive rating of 108 in the minutes Ojeleye was on the court and that actually plummeted to 97.1 without him, so the variance was more on the defensive end.

The luxury for the Celtics is simply being able to deploy another stout body capable of taking some of the wear off Horford. Even if Antetokounmpo scores, Ojeleye can make him work, offer resistance near the basket, and give you another batch of fouls to keep Boston’s top rotation guys on the floor in key spots.

These are the moments that all those medicine ball tosses were building towards. And you can bet Ojeleye will be ready.

After talking one off day at the Celtics practice facility, he promptly disappeared into the weight room. Soon after, the walls to the adjoining media relations office started rattling.

Boom.

Everybody nearby had the same response to the rhythmic thuds. There goes Semi again.

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