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.