The Belgian shotstopper was called upon a number of times as the Black Cats recorded their first away win of the season at Craven Cottage.
"He hurt himself yesterday in training and was a doubt for the game," O'Neill told the club's official website. "It was touch and go whether or not he would be fit, but it was a fantastic performance. I'm delighted with him."
Sunday's victory has lifted Sunderland up into 15th place in the Premier League table.