But he turned in a man-of-the-match performance against Brighton in the fourth round of the FA Cup and earned a rare league start against Manchester United on Wednesday when O'Neill felt the 20-year-old "looked at home".
Now Delph is looking to play back-to-back games for Villa for the first time by being involved in Sunday's fifth-round clash at Crystal Palace.
O'Neill said: "Fabian will have learned a lot from playing against that quality of opposition in United and it will bring him on again.
"He is going to be a proper player and, against a top-notch side, I thought he looked reasonably well at home.
"Fabian is still only 20 but has got a lot of ability, and a lot of potential, and that is what a lot of clubs saw in him.
"Yes, it has been awkward for him. His problem was playing regularly in a side, then stepping up a couple of divisions and playing irregularly in terms of the first team.
"Your general match fitness is affected by that and then, with the opportunities you get, you feel as if you have to doubly impress, that you have to do something special in games.
"But he is going to be a really fine player if he stays clear of injuries."
O'Neill admits that Delph's fearless approach means he will pick up injuries but is reluctant to curb his natural enthusiasm and desire to win.
He said: "Sometimes Fabian goes in for challenges with no fear and, opening up his body like that, he is always likely to get injured as well.
"But at the same time you don't want to curb that enthusiasm and genuine will to win he has that is part of his make-up."
Delph said: "Manchester United are a great side, have great players and they're the champions. For me to be playing against them was a pleasure.
"It was a massive difference to playing against Brighton. Everyone is fit, stronger and more knowledgeable upstairs. You could see that.
"I tried to go in there, work hard and do well. I thought I did that for quite a period of time.
"It was a big boost to be picked for a league game. It shows the manager has confidence in my ability and that does a lot for me.
"I'm still 20, I don't think you can ever stop improving and hopefully I can keep pushing on and it will be brilliant if I am selected Sunday and can play back-to-back games - but it's up to the manager."
Aston Villa 8/11, Draw 5/2, Crystal Palace 4/1