The 58-year-old North Wales man, accused of making monkey gestures, was held after the game at Anfield.
Before the 2-1 win for Liverpool triumphed, the stadium announcer warned fans that any racist and homophobic behaviour would not be tolerated, and it understood life bans could be handed out.
Liverpool has said they will punish anyone indulging in any kind of racist activity and said they were working in tandem with the police to weed out the guilty.
“Following today’s game we were made aware of an image being circulated on social media relating to an incident at this afternoon’s FA Cup tie. We are now working closely with Merseyside Police to establish the facts of what occurred and identify the individual involved,” said an official Liverpool statement.
The FA Cup fourth round match was the first game between the rivals since the eight-match ban handed down to Liverpool striker
Luis Suarez after the FA found him guilty of racially abusing United defender Patrice Evra.
Home fans still booed Evra each time he touched the ball on Saturday, seemingly holding him responsible for being racially abused.
Earlier this month, Oldham player Tom Adeyemi was reduced to tears over what he claimed was racial abuse during the third-round tie at Anfield.
Also at the weekend, Chelsea and QPR footballers were banned from exchanging pre-match handshakes yesterday in an attempt to stop the race row between between players John Terry and Anton Ferdinand boiling over.
The last-minute decision by the Football Association came just days before England and Chelsea captain Terry is due to appear in court accused of racist behaviour.
