Birch's second NBA season saw an increase in playing time as he appeared in 50 games, making his first career start against the Charlotte Hornets on April 10. The 26 year-old averaged 4.8 points and 3.8 rebounds on the season. He appeared in the playoffs for the first time, playing in all five of his team's games. Had his best playoff performance in game five, when he scored nine points, grabbed 11 rebounds, recorded three assists and four blocks. Overall he averaged 5.2 points, 6.2 boards and one block per game in the playoffs. Recorded a double-double on March 14 against Cleveland with 13 points and 11 rebounds in a 120-91 win. He scored 10 points and added five rebounds in a win against Philadelphia on March 25. Birch also made a season-high six free throws against the 76ers, while matching a career-high 10 free throw attempts. He recorded four blocks in a game on two occasions, in a 105-103 win against Boston on January 12 and a 103-83 win against Milwaukee on February 9. He scored eight points and grabbed six boards in 22 minutes against Dallas on December 10.

Following two years overseas, Birch made his NBA debut in the 2017-2018 season. The Canadian carved out a steady role in Orlando, appearing in 42 games and averaging 13.8 minutes per contest as a reserve. Birch quickly established himself as a reliable rebounder, as he averaged 11.2 boards per 36 minutes. Offensively, the 25-year-old worked inside the paint to the tune of 4.2 points per game on an efficient 54 percent from the field. He also shot 68.9 percent from the free-throw line. Birch was a true post player, as he did not attempt a single 3-pointer all season. Defensively, Birch used his length to register 1.3 blocks per 36 minutes. He blocked a season-high four shots in a win over Minnesota on Jan. 16. Birch blocked at least two shots in a game on five other occasions. His season high for points was 13 in a win over Washington on April 11. Birch grabbed a season-high 12 rebounds in a loss to Toronto on April 8. That game was also one of three double-doubles for the rookie.