The Celtics rescinded a qualifying offer to Sullinger last week to make cap room for Al Horford. The Raptors lost Bismack Biyombo to the Orlando Magic in free agency, and Sullinger will serve as a viable replacement alongside Jonas Valanciunas.
The big man's numbers dropped last season, which lead to a drop in playing time. He averaged 10.3 points. 8.3 rebounds, and 2.3 assist in 23.6 minutes. The 24-year-old made $2.3 million as a restricted free agent.
The former 21st pick overall pick had his best seasons from 2013 to 2015, averaging 13.3 points in back-to-back years. He will move to his former team's division rival to find his groove again, north of the border.