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United skipper Harry Maguire played out the ball to Fred, who tried to pass it back to Dean Henderson but instead found Iheanacho.
The 24-year-old Nigerian forward coolly rounded Henderson and tapped the ball into the net for the opener.
Paul Pogba beat Wesley Fofana down the left and played in a low cross, with Greenwood sweeping past Kasper Schmeichel from 12 yards out.
However, Leicester restored their lead early in the second half as Youri Tielemans combined with Iheanacho to score their side's second.
Iheanacho returned late in the second half to complete his brace and seal a 3-1 victory at the King Power on Sunday.
Leicester will now face Southampton at Wembley in the semi-final of the FA Cup.
Meanwhile, the other semi-final clash will feature Chelsea taking on Manchester City at Wembley.
Both semi-final matches will be played in April before the final is held at Wembley on 15th May.
The final of the FA Cup could be played in front of 20,000 fans as part of the proposed events to test the return of crowds in the UK amid the coronavirus pandemic.
Featured Image Credit: PA
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