The Bulls acquired Willie Reed from the Pistons and then waived the center, it was announced. The Bulls and Pistons also traded 2022 second-round picks.
The Grizzlies will get Brice Johnson and a future second-round pick from Detroit.
In yet another deal, the Bulls acquired forward Noah Vonleh and cash from the Portland Trail Blazers in exchange for the rights to Milovan Rakovic. The move delivers a $3.5 million traded-player exception to the Blazers and drops them under the luxury tax threshold.
Chicago acquired Nelson on Feb. 1 in the trade that sent Nikola Mirotic to the New Orleans Pelicans. Nelson will provide depth at point guard for the Pistons, who are without starter Reggie Jackson (sprained right ankle).
Nelson will be reunited with Stan Van Gundy, for whom he played during the coach's tenure with the Orlando Magic from the 2007-08 season to the 2011-12 season. Nelson spent 10 seasons with the Magic and also has played for the Celtics, Mavericks and Nuggets during his career.
The NBA suspended Reed this week for six games without pay because of a domestic violence incident in August.
Ennis, a second-round pick of the Heat in 2013, averaged 6.9 points and 3.5 rebounds in 45 games for Memphis this season, his third with the team. He also had a short stint with New Orleans during the 2015-16 season.
Johnson, 23, a 2016 first-round pick by the Clippers, was averaging 1.8 points this season for the Pistons.