Sir Alex Ferguson has written to all Manchester United fans travelling to Anfield for Saturday's FA Cup fourth-round tie against Liverpool.
The United boss has asked for the fans to behave and for "positive, witty and loud" support.
The letter made no mention of the recent racism row, instead urging fans to help protect future allocations.
Ferguson did the same before the league game in October which resulted in more United tickets for this match.
"I wrote to fans attending the away match in October urging them to co-operate with stewards and officials at Liverpool, so we can make a strong case for restoring our allocation for future United games at Anfield," said the letter from Ferguson.
"The fans did almost everything asked of them that day and, as a result, we have a much improved allocation for this important FA Cup tie.
"Please do everything you can to continue that good work and protect next season's allocation."
Luis Suarez will again be missing for Liverpool as he serves the sixth game of his eight-match ban, after being found guilty of racially abusing Patrice Evra when the two sides drew 1-1 in the Premier League game at Anfield in October.
In the wake of the Suarez saga, the Football Association called on the two clubs to meet before the tie and hold peace talks over any bad blood between the two sides.
Ferguson said at the time that they were uncalled for.
"FA Cup ties are tense affairs at the best of times," his letter continued.
"Add in the fact that Manchester United against Liverpool is the biggest game around and it becomes even more so.
"Your support is vital to the team and down the years that has been especially true at Anfield."
Merseyside Police match commander, chief superintendent Jon Ward, said the behaviour of the "majority of the fans" at that last meeting at Anfield in October between the two sides was very good.
"We know that this match will be a busy operation," he said. "We hope that the fans will continue to act as ambassadors for their clubs and their cities and enjoy what I am sure will be a good game.
"We work closely with both clubs, Greater Manchester Police and the Football Supporters Federation and, if it's brought to our attention, any inappropriate remarks and behaviour will be acted upon."