The Red Devils boss is keen for the club’s supporters to act responsibly when they face their bitter rivals on Saturday, hoping to fully restore their Anfield ticket allocation
The Red Devils ticket allocation for the clash has been increased following improved behaviour by United’s fans when they visited Anfield for a Premier League match earlier in the season.
However, Saturday's match will be the first between the two teams since Liverpool striker Luis Suarez was handed an eight-game ban for allegedly racially abusing Manchester United’s Patrice Evra.
Despite not mentioning the incident between the two players in a letter sent to United fans set to attend the game, Sir Alex is aware of the need for excellent behaviour off the pitch by supporters.
The letter read: "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.
"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 by: Going to the seat that you've bought, not blocking aisles and gangways, respecting the stewards and following the ground regulations.
"FA Cup ties are tense affairs at the best of times," he added. "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. But please put the emphasis on getting us into the next round and giving the sort of support you are famous for - positive, witty and loud."