The site encouraged visiting supporters to "ridicule their abhorrent adversaries" ahead of tomorrow's visit to Elland Road.
Leeds' Ian Silvester said: "We took the matter up with Manchester United immediately and asked them to consider withdrawing the statement.
'It was disappointing to read something like that, given the historical background to the fixture. We hope everything passes off without incident, but comments like that don't help.'
Leeds United Independent Fans Association chief Simon Jose added: 'It's completely unnecessary and shows a lack of sense. It seems nowadays football fans have a sense of good behaviour while players and clubs don't.'
The attack was immediately deleted after the Yorkshire Evening Post and Leeds United brought it to the attention of officials at Manchester United.
Leeds legend Peter Lorimer was alarmed by the comments: “This is brainless and totally irresponsible, an absolute outrage.
“Everybody in football knows the sensitive nature of this particular fixture. They are always highly-charged encounters when Leeds and Manchester United clash.
“It’s been that way for many years now, even going back to the days when I played in the 60's.
“Just recently, though, it has to be said the fixture has become a particularly tense affair.
“The police are very much aware of that and that is why they take appropriate measures for policing every game.
“But anything like this which threatens to upset the apple cart even more simply isn’t needed. It is absolute stupidity that some individual should do something like that.”