The Scarlets captain and team-mate Rhodri Jones face a disciplinary hearing after a dangerous tackle on the Ospreys forward, who represented the Lions in 2005, during the Boxing Day victory over their local rivals.
If found guilty the players could face a suspension of anything between three and 52 weeks according to International Rugby Board regulations.
Rees played for the famous tourists eight times in South Africa two year's ago including starts in all three Tests against the Springboks.
Both players along with Tavis Knoyle, who requires shoulder surgery which will rule him out of the Six Nations, will miss tomorrow's RaboDirect Pro12 clash with Newport Gwent Dragons.
Head coach Nigel Davies has made14 changes to the side that beat the Ospreys as the Dragons target an eigth consecutive win over the Dragons and a second Welsh derby triumph on the bounce.
"We don't underestimate the size of the task ahead of us on Friday night but neither should anyone underestimate the competition for places within our squad and how close some of the calls on selection currently are," said Davies.
"We are clear as a squad on how important these games are for our season. We have respect for the Newport Gwent
Dragons; whenever you go up to Rodney Parade you're aware they have a very good home record, they will be physical and manage the contact area well and will be expecting to win in front of a home crowd."