Daniel Theis has successful surgery and will be sorely missed by the Celtics

“I just tried to bring some energy and have fun out there,” Daniel Theis said after his first game in a Celtics uniform, a 12-7-3 preseason showcase. The pace of play got to him during this period, but by Nov. 1 he said he was caught up, “it’s still basketball,” he said, comparing the NBA game to his European experience.

Games before Opening Night don’t often showcase what’s to come, but this time, it did. That happened, in part, thanks to Gordon Hayward’s absence and the playing time it opened up. Unfortunately, now they’ll unite in recovery from tragic injury. Hayward suffered his six minutes into the season, Theis with under one minute remaining in Sunday’s loss. Theis didn’t play against the Cavaliers when he watched Gordon Hayward break his ankle from the sideline. He was one of the first Celtics to visit him in his home after season-ending ankle surgery. Hayward, in turn, helped coach Theis through the first weeks of his NBA career.

On Sunday night, the team announced that Theis had undergone successful on his left knee lateral meniscus and should be ready for training camp next season, but even with Greg Monroe in tow, he’ll be missed heading into this year’s playoffs. Through 18 regular season games, Theis only averaged 12 minutes. While the Celts ripped off 16 straight wins on the way to a 16-3 start, he flashed promise, including a 10-10 double-double against the Kings on Nov. 1, but often found himself playing fewer than 10 minutes.

Hayward noticed, telling the Globe’s Adam Himmelsbach, “he’s basically taking a pay cut to not be the best player on his team in Germany, and come over here and sit on the bench.” So he took Theis under his wing, and while he never commanded the coach role that the Celts envisioned him becoming after his injury, he helped mentor Theis into one of the more impactful rookies in basketball this season.

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Theis’ role evolved over the regular season. His ability to keep up with the speed of the NBA game after a short adjustment period garnered him four more minutes per game after Nov. 24. He caught both lobs and rolls to the rim at a 74% efficiency, and then began to flash his three-point shot, which he didn’t use often but converted at a 40.6 percent clip since New Year’s. He also established himself as a defensive menace, with a 3.5 defensive box plus-minus that’d rank in the league’s top-five if he qualified.

His ability to play with pace in space and switch defensively quickly made him a Stevens’ favorite. Even in a crowded front court that included Marcus Morris, Al Horford, and Aron Baynes, he became a common and effective contributor on both ends of the floor. With starter Baynes on the mend for a game against the Knicks on Feb. 24 with elbow soreness, Theis started instead of the newly-signed Monroe and contributed six points and seven rebounds in a +9 effort.

Since Monroe’s debut on Feb. 8, Theis continued to average more minutes (17.8 to 15.2) and posted a net positive while Monroe put up a negative rating in seven of his first 11 games. That said, Monroe outscored, outrebounded and trended in an optimistic direction, with positive +/-’s in three of his last five outings. The backup big role is now all his with Theis on the mend.

It’s hard to compare the two in terms of value. Monroe is the more polished player on offense, but Theis is the better fit defensively. Twenty-seven fans even put together this Change petition to reveal Theis’ official wingspan because of his prowess of defending pick-and-rolls and quickly getting to shots. Even though Theis posted a top-10 rebounding total among rookies while ranking only 30th in minutes, Monroe beats him in that category per-minute.

According to the small sample returns on Monroe, the Celtics have maintained a 100 possession per 100 minutes pace with Monroe on the floor. That’s right around Theis’ 97.75, but with 52 fewer games to go off. Pace, despite other concerns, could be maintained with Moose. Stevens will get 0 three-point attempts from Monroe, as he’s attempted 12 total in his career. That alone is a loss, but his effective passing from the low block may overcome his deficiencies from the perimeter. Ultimately, on a team level, losing Theis eliminates a match up card for Stevens. All year, he’s touted Boston’s ability to line up based on who they’re playing. Now, his options in the front court are more limited.

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To replace Theis in the aggregate, Boston’s recalled Guerschon Yabusele from his G-League stint and Stevens hinted at minutes for Semi Ojeleye guarding skilled fives. Neither have contributed substantially, unless you’re counting Yabusele’s meme-rating (definitely high), but Stevens has driven minutes Ojeleye’s way. He’s shown promise as a defender but has not been able to drill open threes (28.7%), basically playing offense as an exclusively spot-up three-point shooter who can’t hit them.

The Celtics’ most important force in the front court, Al Horford, is only dealing with an illness at the moment but his health is paramount for the team’s playoff standing. Horford didn’t play much with the second unit because when the bench took the floor, Theis’ shooting, defensive range, and ability to dribble in the high post and hand the ball off he provided Boston’s bench a pseudo Horford.

He’ll be back next year, once again at a bargain salary of roughly $1-million, but his heightened value on this team starving for depth of production to make up for Hayward’s loss will reveal itself as tough to swallow moving forward.