The Red Devils came from two goals down to edge the Tigers 3-2 at the KC Stadium which leaves them seventh on the Premier League table, eight points off league leaders Arsenal.
Rooney, who scored United's second goal, spoke of the magnitude of the victory which could really kickstart the campaign for the reigning champions.
"We didn't start well enough today. Two-nil down we had to dig deep and show our fighting spirit. We've done that and this is a massive result for us," Rooney told Sky Sports 1.
He added: "It was just poor defending. It was my fault for the first, I've let James Chester go, and then a sloppy goal.
"We had to come back and try and get an early goal after they scored the two."
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