Williams had an MRI on Sunday, but the test only revealed damage, according to the Salt Lake Tribune. Williams suffered the injury midway through Saturday's loss to the Miami Heat. The Tribune reports that veteran Jamaal Tinsley will take Williams' starting spot.
Tinsley played well in place of Williams on Sunday night, scoring 11 points and handing out four assists in a 97-93 win in Orlando.
"It is tough, you don't wish that on anybody," Tinsley said of Williams. "This is just the nature of the beast. I get another opportunity to go out there and play. We want him to get well and come back soon. This is just the NBA. Somebody goes down and somebody else get the opportunity to play. ... We just have to figure out a way to win ballgames and do the right thing."
Williams was hurt late in the second quarter and did not return for the second half of Utah's 105-89 loss.
An X-ray taken on Williams in Miami was negative, but the 10-year veteran said the pain in his thumb was similar to what he felt when he tore a ligament in his hand in 2008.
"I can't feel it now," Williams said after the game of the numbness. "It's the same feeling (as in 2008). I'm just crossing my fingers that the MRI is negative."
The Jazz immediately felt the impact of the absence of Williams, who averages 13.3 points and a team-best 6.8 assists per game this season. Utah, which trailed 47-43 at the half against Miami, was outscored 58-46 in the second half and also surrendered 20 points on 11 turnovers over the third and fourth quarters on Saturday.
"Mo is the guy that drives the pace for us," coach Tyrone Corbin told the Tribune. "We're going to miss him, we'll see how long this thing is, we don't know yet until he gets looked at by Dr. Mason when we get back, but hopefully it's shorter than longer."
Information from ESPN.com's Michael Wallace and The Associated Press was used in this report.