Williams, a 6’10″ center/forward, appeared in eight games for the Dallas Mavericks this season averaging 3.6 points and 1.6 rebounds per game.
O’Neal had season-ending wrist surgery late last month.
Williams, who spent three seasons at Boston College, appeared in 27 games for the Texas Legends of the NBA Development League, averaging 15.2 points, 8.9 rebounds and 2.5 blocks. Williams was originally drafted by the New Jersey Nets with the 17th overall pick in the 2007 NBA Draft and has posted career averages of 4.3 points, 3.4 rebounds and 1.2 blocks in 134 career games split between New Jersey and Dallas.
He is the 55th call-up from the D-League this season, a record.
Williams is expected to be in uniform tonight in Atlanta when the Celtics play the Hawks.