But he made a costly error in the FA Cup clash with Blackburn and was also uncertain during the Carling Cup semi-final second-leg clash with Sam Allardyce's side.
O'Neill would not reveal whether he would continue his policy of playing Guzan in cup ties ahead of Brad Friedel in this weekend's tie with Brighton.
But his faith in the former Chivas USA player, currently number two to Tim Howard in the United States squad, remainds undiminished.
O'Neill said: "Do I still have faith in Guzan? I wouldn't want to be commenting in terms of the performance of players. I would kind of leave that to the side.
"But I do know that Brad Guzan, in my view, will be a real quality goalkeeper.
"It is always difficult for a player coming in and needing to perform when you haven't had a host of games behind you. That is difficult.
"But his performance in the Sunderland game was fantastic. It was a big reason why we got to the quarter-finals - and now we are in the final
"But there is no point in you asking if he will play on Saturday."
O'Neill has revealed he will make several changes for the cup clash with Emile Heskey (foot), Richard Dunne (hamstring) and skipper Stiliyan Petrov (achilles) all doubtful.
He said: "I will change the side around. Playing three games in six games has taken its toll and I've got another group of players I am going to rely on to get us through this tie.
"We've got one or two younger players around that did well in the Peace Cup in the summer (in Spain) and who are desperate for a chance and some who are currently unlucky not to be in the team.
"But I will make them fully aware of Brighton. I saw them last week against Walsall and they played well in really heavy conditions. If they perform like that, they will cause us problems.
"We've got the home tie but Brighton are bringing around 6,000 fans and it will be a tough afternoon for us.
"But with that result and performance against Blackburn, and everything that game entails, whoever takes the field on Saturday should be ready - whether they have played the last couple of weeks or not."
O'Neill has dimissed reports that he wants to sign Rangers striker Kris Boyd.
He said: "I was quite surprised to see the story that he is signing for us.
"Of course, I know the player very well indeed. He was in Scotland when I was in charge of Celtic - but there isn't any truth in it."
O'Neill is expecting Villa midfielder Craig Gardner to move to Birmingham in the next few days.
He said: "I think it will get done. In terms of communication, I haven't heard a great deal from Birmingham but they are not a million miles away from agreeing things."
Aston Villa v Brighton 1st Goalscorer: Albrighton, M. 16/1