Benitez, who has failed to win over sceptical Chelsea support, seen lingering title ambitions quashed, crashed out of the Champions League and is now in a desperate scramble to secure a top four berth.
And now he faces a trip to Old Trafford for Sunday's FA Cup quarter-final.
"I'm not going to kick anyone when they're lying down," said Ferguson.
"Chelsea have a terrific record in the FA Cup, probably the best of any team in the last decade.
"It doesn't matter when you play Chelsea or who's in charge, we'll have a hard game."