Riley controversially denied United a 68th-minute penalty that would have broken the deadlock against David Moyes' Toffees, after Everton defender Phil Jagielka appeared to clip Danny Welbeck inside the danger-zone.
“When he sees it, he will probably realise he’s made a mistake," said Ferguson. "I can’t say that’s the reason for us losing entirely but I couldn’t see Everton scoring."
And Welbeck - at the centre of the incident - sided with his manager.
“I was barged on my left-hand side and as I went down I was caught on the leg as well," he said.
“I had no reason just to go down because if I had not been fouled I would have scored. I was round the keeper. I’m disappointed to lose. We should have won even without that penalty because we were the better team.”
It was Jagielka himself who later ended the Reds' FA Cup hopes, converting the winning penalty in the shootout after ex-United goalkeeper Tim Howard had saved both Dimitar Berbatov and Rio Ferdinand's efforts.
And 26-year-old admitted that he was expecting the penalty to be given.
“I touched him, but I’m not sure how much that caused him to go down," he said.
"Thankfully the penalty was not given. Maybe that was a bit of luck but we will take that.”
NEWS: Moyes - United should have had penalty
EVERTON FANS! Check FourFourTwo.com’s ever-expandinginterviews archivefor more stuff to read:
Perfect XI Graeme Sharp
What Happened Next? Norman Whiteside
Plus Graeme Sharp on Weekend Wonders and Everton on Talentspotter
UNITED FANS! Check FourFourTwo.com’s ever-expandinginterviews archivefor more stuff to read:
Web Exclusives Wayne Rooney, Peter Schmeichel, Carlos Tevez
Q&A Wayne Rooney, Cristiano Ronaldo, Ruud Van Nistlerooy
One on One David Beckham, George Best
Ask A Silly Question Bryan Robson
Boy's A Bit Special Danny Simpson
Perfect XI Garth Crooks, Bryan Robson, Sir Bobby Charlton, Eric Cantona
What Happened Next? David May, Norman Whiteside
Sing When You’re Winning Dominic Monaghan, Ian Brown
And Another Thing Being a Big Four fan
Plus! Rate United players (and others) on
