The injury will force Yi to miss at least an additional week after he was expected to return Sunday against the Knicks or Tuesday at Chicago. He has missed five weeks with a sprained right knee.
Reports differed on whether the injury happened during a three-on-three or two-on-two drill, but this much was known: Yi went up to contest a dunk attempt by the 6-foot-10 Williams, the two collided as Yi got a piece of the ball and Williams' dunk attempt missed.
Witnesses said Yi wanted to continue playing but was told to seek medical attention.
"I would have never thought it was something like that," Williams said of the number of stitches required to close the wound. "We were just playing, out there trying to get better, and I looked up all of a sudden, and his mouth was bleeding. Now y'all are telling me he needed 50 stitches, and I'm like, 'Really?'"
Yi, a third-year forward from China, was averaging 9.8 points and 7.5 rebounds before he was injured in the third quarter of a game at Charlotte on Nov. 2.
He has been reunited with new Nets assistant coach Del Harris, who joined the staff as Kiki Vandeweghe's lead assistant on Thursday after the firing of Lawrence Frank. Harris was Yi's coach on the 2004 Chinese Olympic team.
Chris Sheridan covers the NBA for ESPN.com.