The Portugal international was given his marching orders for a high challenge on Alvaro Arbeloa on Tuesday night, when Madrid clinched a place in the quarter-finals of the elite competition courtesy of a 2-1 win.
"Quite simply, its hard to keep your faith in the game," Ferguson said in a press conference. "It's the third time we've been knocked out by a referee's decision, it's not easy to take.
"But in the analysis of the cold of the day you have to realise that for Manchester United, we just have to get on with it. Nothing can be done now. I've cleared my mind now, it's just another day in the history of this club, not a good day but another day."
Meanwhile, the Scotsman has insisted that reports that striker Wayne Rooney will be sold in the summer are "absolute rubbish" and that he was left out of the starting XI against Madrid for purely tactical reasons.