Williams has been cited under Law 10.4(a) 'A player must not strike an opponent with the knee(s)' and faces between a three and 52-week ban if found guilty.
A three-week suspension would leave Williams unavailable for Friday's Heineken Cup trip to Castres, as well as the Magners League Welsh derby with the Scarlets and the Round 7 fixture at Aironi, while a mid-range ban of eight weeks would prevent him from playing any part in the autumn Test series.
It would be a major blow for both club and country if a ban was dished out, but both sides are better prepared for the potential absence of Williams with the emergence of Sam Warburton.
Wales also have a number of other contenders for the No7 shirt: Gavin Thomas was the last person to play on the openside flank for Warren Gatland's men having started both Tests on the summer tour to New Zealand, while Rob McCusker sat on the bench against the All Blacks and has found some fine form with the Scarlets.
Scarlets flanker Jonny Fa'amatuainu was also cited for a dangerous tackle after this weekend's European action, as was Clermont Auvergne's Jamie Cudmore and Munster centre Sam Tuitupou.
All four players will stand appear at a European Rugby Cup disciplinary hearing in Dublin tomorrow (Wednesday).