Although United head to the Etihad Stadium three points clear of their neighbours, who were dumped out of Europe altogether on Tuesday, Ferguson is adopting a cautious approach.
In his programme notes ahead of Wednesday night's 1-0 defeat to CFR Cluj, Ferguson outlined his belief that both sides may opt to play safe.
The United boss is particularly fearful of his leaky defence.
And it leads him to conclude it would be a significant moment in his Old Trafford tenure if the Red Devils were to come out on top.
"If we win [on Sunday] it will be one of our best ever results," Ferguson told MUTV.
"They are a really good, powerful team with massive players.
"It won't be easy and if we defend like we did at Reading we'll be in trouble.
"The important thing for us is to take lessons from the mistakes we're making and do something about it. We need to find a solution."