The 29-year-old was sent off during Ospreys' 36-3 defeat to Leinster in the Heineken Cup on Friday.
An ERC statement said: "The independent judicial officer, Jeremy Summers, heard evidence and submissions from Evans, who pleaded guilty, from Ospreys team manager Andy Lloyd and from the ERC disciplinary officer.
"In upholding the red card decision, the judicial officer stated that the incident involved at least two stamps which connected with McCarthy's head and which resulted in a serious facial wound.
"Mr Summers also stated that there was no place in the game for conduct of this kind — a view which Evans himself accepted during the hearing."
A 16-week ban was initially put forward, but was reduced by four weeks due to Evans's guilty plea.
Evans can appeal the decision.