Theis started the 2020-21 regular season with the Boston Celtics but was traded to the Bulls on Deadline Day in the deal that sent Moritz Wagner the other way around. The big man played in 23 out of possible 29 contests with the Bulls and experienced an uptick on his role as the season progressed, as he came off the bench in his first nine appearances but later moved to the starting five and never looked back. Theis averaged 9.5 points and 5.2 rebounds in 24.4 minutes per game across 42 games as a member of the Celtics, but those numbers received an uptick since the midseason trade -- he averaged 10.0 points and 5.9 boards in 25.0 minutes per game with Chicago. Theis' best scoring output of the season came on April 26 against the Miami Heat when he finished with 23 points and 12 rebounds, and that would be one of two games where he ended with that 23-point mark. He had three games with at least 20 points and 13 appearances where he recorded at least 15 points. Other season-high marks for the big man include 12 rebounds, six assists, three steals (three times) and four blocks. He delivered one of his most complete games of the season on January 25, when he finished with 19 points, three rebounds, three assists, three blocks and three steals across 28 minutes against the Bulls. All things considered, Theis ended the 2020-21 season averaging a career-high 9.6 points per game while shooting 54.1 percent from the field, 32.2 percent from three-point range and 67.3 percent from the free throw line. He also averaged 5.5 rebounds, 1.7 assists, 0.9 blocks and 0.6 steals per tilt.

After spending his first two NBA seasons in a bench role for the Celtics, Theis stepped up into far more responsibility in 2019-20, starting 64 of his 65 games in the pandemic-shortened campaign. The returns were all positive, as Theis set career highs in practically every category. His counting stats jumped up to 9.2 points, 6.6 rebounds, 0.6 steals and 1.3 blocks per game. And his efficiency improved too, as Theis set career highs in both field-goal percentage (56.6) and free-throw percentage (76.3). The 6-foot-8 center opened the season on a shot-blocking spree, piling up 17 swats in his first eight games, including a career-high five on Nov. 5 against Cleveland. Theis went on to contribute double-digit points on 31 occasions and record seven double-doubles. Twice Theis dished a career-high six assists, which included an eight-point, 14-rebound, six-assist effort on Nov. 20 during a road loss to the Clippers. In his biggest game of the year, Feb. 21 against Minnesota, Theis exploded for 25 points and 16 rebounds -- both career highs. Once playoff time rolled around, Theis remained in the starting lineup for all 17 games of Boston's run to the Eastern Conference finals. He double-doubled with 13 points, 15 rebounds and a pair of blocks in a Game 1 win in the second round against Toronto, later adding a 15-point, 13-rebound double-double with three blocks in Game 5 of the conference finals against Miami.

For the second consecutive season, Theis provided impressive scoring, rebounding and defensive ability in a very limited role for the Celtics in 2018-19. He posted 5.7 points, 3.4 rebounds, and 0.9 combined steals and blocks per game despite actually seeing his minutes reduced from the prior season (from 14.9 to 13.8). His per-36-minute averages of 14.8 points, 8.9 rebounds and 1.7 blocks again suggested that he was deserving of additional playing time. As he had in his rookie season, Theis shot very efficiently, delivering a 54.9 field-goal percentage, and he became far more accurate from long range (38.8 percent). The 6-foot-9 German missed eight of Boston's first 15 games due to foot injuries, but he returned to serve as a reliable backup at center and power forward. In a Dec. 8 blowout win at Chicago, Theis scored a career-high 22 points to go with 10 rebounds, five assists and four blocks. That was also his only double-double of the year. Theis went on to appear in seven of the Celtics' nine playoff games, averaging 1.7 points and 1.4 rebounds over just 6.0 minutes per game.

After seven seasons of Bundesliga and EuroLeague basketball, Theis signed a two-year agreement with the Celtics ahead of the 2017-18 season. The 25-year-old NBA rookie appeared in 63 contests for Boston, starting in two of them while providing defensive grit and energy off the bench. Despite averaging just 14.9 minutes per game, Theis put up averages of 5.3 points, 4.3 rebounds, 0.8 blocks and 0.5 steals -- good for per-36 averages of 12.7 points, 10.5 boards,1.8 blocks and 1.2 swipes. He was efficient both from the field (54.1 percent) and from the line (75.3 percent), and Theis even showed some long-range shooting ability, sinking 0.3 threes per game at a 31.0 percent clip. He picked up his first NBA double-double Nov. 1 against Sacramento, collecting 10 points and 10 boards in just 18:18 on the court. On Dec. 23, Theis set his season high in rebounds with 15, adding 10 points for another double-double. On Feb. 23, Theis scored 19 points with seven boards, two steals and two blocks in just 20:55 on the court. On Mar. 12, Theis underwent season-ending surgery to repair a torn meniscus in his left knee, which caused him to miss the Celtics' deep postseason run to the Eastern Conference Finals.