Slater was caught up in an incident close to Madrid's Bernabeu stadium in which police wielding batons allegedly charged City supporters in an unprovoked attack prior to kick-off.
As well as being knocked out, Slater, a Manchester City fan, was left with minor head injuries and severe bruising.
He has now reported the incident to the head of security at the Football Association and expects the matter to go to UEFA.
Slater told Charlton's official website: "I can confirm that I was assaulted by at least one police officer in Spain without any provocation at all.
"I have been in contact with the FA, and I am expecting them to take the matter up formally with UEFA."
City lost 3-2 to Madrid in the Champions League clash on Tuesday, September 18.