The Celtics selected Allen with the 53rd overall pick of the 2017 draft. He appeared in 18 games, tallying 19 points, 11 rebounds and 12 assists in 107 total minutes on the floor.
The former Arizona Wildcats star spent most of his rookie season with the Maine Red Claws of the NBA G League. He averaged 17.7 points and 5.0 assists but shot 30.0 percent from three-point range.
Allen also appeared for Boston in the 2018 NBA Summer League but struggled to make an impact. He shot 7-of-18 from the field and averaged 2.8 points in five appearances.
The Boston Globe's Gary Washburn reported Saturday the Celtics planned on releasing Allen and signing another player on a two-way contract to replace him.
Allen's defense often set the tone for the Red Claws. According to NBA.com, he had a 95.3 defensive rating in 15 wins and a 113.3 defensive rating in 19 losses.
Another team should give Allen a look on a two-way deal, but he may need to prove himself in the G League—particularly with regard to his shooting—before he warrants a regular spot in an NBA rotation.