The Dutchman denied a charge of misconduct after saying that ''every aspect of the match'' had been ''against'' his team following United's surprise fourth-round goalless draw at Cambridge last month.
An FA statement read: "Following an Independent Regulatory Commission hearing on Wednesday 18 February 2015, Louis van Gaal has been warned as to his future conduct after he was found to have breached FA Rules in relation to media comments.
"The Manchester United manager denied that his comments which followed The FA Cup Fourth Round tie against Cambridge United on 23 January 2015 alleged and/or implied bias on the part of the match referee, and/or brought the game into disrepute."
Having seen the Barclays Premier League giants held by their League Two hosts in the initial encounter between the sides on January 23, Van Gaal said: "Every aspect of the match is against us - the pitch, the referee, everything that you can think about is against you.
"You have seen the referee
It's always the same
Everywhere I have coached these games - and I have coached them also with other clubs - it's always the same.''
Van Gaal expressed surprise at the FA charge and immediately announced his intention to contest it.
He said: "I am very disappointed
For nearly 30 years I have been a trainer, coach or manager and I have never been charged.
"Up to now I don't think I said something wrong
I said in our press conference the same phrases, because I know in advance that everything is in favour of the underdog
It is always like that."
Although Van Gaal was found to be in breach of the rules, the fact he has avoided a fine or a ban suggests the panel deemed it a relatively minor indiscretion.
United won the replay 3-0 at Old Trafford and set up an FA Cup quarter-final against Arsenal by seeing off League One Preston 3-1 on Monday night.
Source : PA
Source: PA