Sources told ESPN.com that the former Boston College center, waived by the Dallas Mavericks in March, is scheduled to join the Celtics in Atlanta and will be in uniform for Boston's game on Friday night against the Hawks.
To make roster for Williams, Boston waived the injured Jermaine O'Neal on Thursday night, according to sources aware of the transaction.
Williams joins former Cleveland Cavaliers center Ryan Hollins as new big men brought in by the Celtics late in the season to replace O'Neal and Chris Wilcox in support of a resurgent Kevin Garnett (who has played primarily center this season) and rookie surprise Greg Stiemsma.
O'Neal underwent season-ending wrist surgery late last month. Wilcox was waived on March 23 after being ruled out for the season due to a heart condition.
Williams was released on March 22 by the Mavericks, who decided they needed another wing player instead of four centers and used the vacated roster spot to sign swingman Kelenna Azubuike. Because Williams was waived by Dallas before the league's March 23 deadline, he is eligible to play for Boston in the playoffs.
Williams cleared waivers on March 26. The 6-foot-10, 235-pounder averaged 3.6 points, 1.6 rebounds and 8.1 minutes in eight games with the Mavericks but averaged 15.2 points, 8.9 rebounds and 2.5 blocks in 27 games for the Texas Legends, Dallas' D-League affiliate.