It came at the beginning of a vital week where Villa face CSKA Moscow in the UEFA Cup last 32 on Wednesday and Chelsea in the league on Saturday, with both games at Villa Park.
A tremendous season could begin to unravel if Villa cannot recover quickly from the defeat at Everton.
And Knight said: "We have CSKA Moscow and then Chelsea at home this week. They are two crucial games for our season.
"We have to bounce back quickly from the disappointment of going out of the FA Cup. We have to get over this, and make sure we continue our good season.
"The cup defeat has made us even more determined to hit straight back with two difficult home matches. We had not lost for a long time and it wasn't a nice feeling.
"And we conceded three goals, when we have been keeping a lot of clean sheets over the weeks so we must get back to that standard.
"We must go back to Villa Park and show the fans that we still want success this season, we still want to have a good conclusion to the season.
"Moscow will be a difficult game. But we must pull ourselves together after this and show what we are made of."
Knight reckons Villa deserved more from their trip to Goodison Park, saying: "It was a bit crazy at the beginning with three early goals, but we feel that after the break we deserved something from the game.
"It was all very frustrating. To concede the third goal like that maybe showed a lack of concentration.
"I must admit, though, that their front two were always a handful. Victor Anichebe is a tough opponent.
"There has been a lot of speculation about him in the last few weeks, with the prospect of a move to Hull and his problem with the (Everton) manager (David Moyes).
"But he came back into their team and proved his manager wrong. He and Tim Cahill are a real handful, together they made it very difficult for us."
O'Neill also believed that Villa deserved more.
He said: "I feel we are unlucky not to be in the last eight. We missed lots of chances and we were forcing Everton back when they got out to score their third.
"They are strong, and they have had a brilliant time. But I felt we were never out of the game until they got that third.
"There was a hostile crowd, it was frenetic. But we were well in the game.
"But I was delighted with the team, as I have been all season. They have brought so much to the club this season, we are still strong and hard to beat.
"Everton won and good luck to them, and they are more than capable of winning the competition.
"They are in great shape and play with the same spirit and effort that we do.
"But I do not want to lose our momentum now. We want to qualify for Europe and it is important to keep going."