The Manchester United's irritation with Chelsea counterpart Rafael Benitez goes back to the Spaniard's days at Liverpool and that infamous rant in 2009, when he claimed it was a "fact" that Ferguson manipulated referees and received preferential treatment from the Football Association.
Ferguson has never forgotten the attack and when Benitez turned up at Chelsea following the dismissal of Roberto Di Matteo earlier this season, he labelled the former Valencia boss as "very lucky" for being given the chance to take over two Champions League-winning teams, having also replaced Jose Mourinho at Inter Milan.
It has not proved so lucky for 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.
Benitez has railed against the club's hierarchy for naming him an "interim manager" and he now accepts there is no chance of getting the job on a full-time basis.
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. "It's not my style. My concern is Manchester United. What happens at Chelsea doesn't really affect me.
"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."
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