Wayne Rooney is out to deny Manchester United a record-equalling FA Cup triumph.
The striker, United's record scorer, faces his former club with Derby County in the fifth round tonight for the first time since returning from an 18-month spell at DC United.
Ole Gunnar Solskjaer's side travel to Pride Park unbeaten in their last eight games in all competitions and chasing a Champions League place.
They are also hunting a 13th FA Cup to draw level with Arsenal's record haul but Rooney wants to end their hopes.
"My time at Manchester United, I loved every minute of it. I am a Derby County player now and I want to win with Derby," said England's 53-goal record scorer.
"For that 90 minutes, or 120 minutes, obviously I want Manchester United to lose.
"Everyone will understand that I want Derby to win in this game, and after that I will go back to being a Manchester United fan and wanting them to win.
"The one team I wanted was Manchester United. We were on the coach, actually, going to the hotel the night before a game when the draw was taking place and there was a cheer when United came out. It is a great draw for us."
Rooney (34) spent 13 years at United, scoring 253 times, before returning to Everton in 2017.
He won every domestic honour, along with the Champions League and the Europa League, during his time at Old Trafford.
Phillip Cocu's Derby are 13th in the Sky Bet Championship and while Rooney knows the Rams are underdogs, he believes they can cause a shock.
"For us as a team it is a great challenge for us to come up against Manchester United, to see if we can beat them. It is a challenge but it is something we are all looking forward to," he told United's website.
"This is a game that is a big opportunity for us to go into the quarter-finals of the FA Cup.
"Obviously for Manchester United, there is this and the Europa League where they have a chance to win a trophy this season, so we know that they won't be taking it lightly."
United boss Ole Gunnar Solskjaer is wary of the threat Wayne Rooney poses to his side's FA Cup hopes as the Derby forward looks to prove there is "still fight in an old dog".
"He gave absolutely everything for this club and we all appreciate that," Solskjaer said.
"He's a threat - in and around the box, set plays - and of course this game Wayne will show what he can do and he wants to prove that there's still fight in an old dog.
"We've got to be on our toes, don't give him any space in and around the box or in midfield.
"At the latter end, when he played in midfield, you can see he's got Scholesy-like passing range."
Solskjaer praised former team-mate Rooney's personality as well as his on-field performance, revealing that he was the favourite player of his son Noah.
But it feels like England's all-time top scorer is under-appreciated in some quarters, albeit not by the Old Trafford faithful.
"He's been a fantastic player for Man United and he played here for so long, he's the club's top scorer, he's won so many trophies," Solskjaer said. "For me, I don't think the fans underrated him. I think everyone really, really appreciates what he did for us."
Derby County vs Man United
Emirates FA Cup Fifth Round
Pride Park, Tonight, 7.45pm