Manchester United and Liverpool have issued a joint statement condemning the fans who attacked the ambulance taking Alan Smith to hospital at the weekend.
Smith faces months out recovering
Smith, 25, suffered a broken leg and a dislocated ankle during United's 1-0 FA Cup defeat at Anfield.
The clubs' statement read: "These people aren't real fans of football.
"The actions of a few idiots should not overshadow the efforts of all involved to make sure Alan received the best treatment as soon as possible."
Reports in several newspapers on Thursday claimed the ambulance was surrounded by people who threw stones and bottles at it and even tried to rock it from side to side.
The two paramedics - both Liverpool fans - were described as being "horrified" by the hostility shown once the culprits realised the occupants of the ambulance were Alan Smith and Manchester United's club doctor Mike Stone.
A spokesman for the Merseyside ambulance trust confirmed the attack: "It did not result in any delay in transferring the player to hospital but the trust cannot condone this type of behaviour while administering emergency treatment."
Smith, who later underwent surgery on the injury, was given a general anaesthetic before he left the stadium so was not aware of the attack on the ambulance.
Les Lawson, secretary of the Merseyside branch of the official Liverpool Supporters' Club, told BBC Radio Five Live: "The Liverpool supporters who were actually inside the ground gave Alan Smith a standing ovation as he left the field.
"As far as I am concerned, the people who attacked the ambulance are not genuine Liverpool supporters because you just don't do that sort of thing.
"You never know when you are going to need the paramedics yourself or for a member of your family, so how would they feel if mindless idiots came out and tried to shake an ambulance and throw missiles.
"These are just not genuine football fans. Words just fail you really.
"I don't think the relationship between the two sets of fans will ever improve really because there is a great rivalry and the fans just don't like each other," he added.