Jackson, 26, is 6-foot-8 and signed a 10-day hardship deal with the Celtics on Dec. 18, appearing in one game.
He was the No. 15 overall pick by the Kings in the 2017 NBA Draft, spending most of last season with the Thunder, appearing in 34 games and averaging 7.2 points.
He also played one game with the Bucks last season, and was waived by the Mavericks before the start of this one in training camp. Jackson spent the first part of this season with the Texas Legends of the G League.
Terry, 25, is 6-9 and appeared in three games with Phoenix.