"The update on Mario is he had a procedure done to relieve some discomfort in his wrist and we expect him back for practice next week," Gailey said per BuffaloBills.com. "That’s all I know at this point."
It's not known whether Williams had the procedure done today or earlier in the week, but the fact that the team expects Williams on the practice field when they re-convene next week indicates that the procedure was fairly minor. The "discomfort" Gailey alludes to is a wrist sprain, an injury that the NFL itself revealed upon fining the team $20,000 for not disclosing the injury in the early portions of the regular season.
We may eventually get more details about what exactly happened with the wrist "procedure" - perhaps even from Williams himself - but for now, this issue appears to be laid to rest. Or, rather, it's laid to rest until the next time Mario resuscitates it.