And the combative Delph, who could come back into Villa's plans for today's FA Cup quarter-final trip to Reading, is having extra training to improve his technique.
O'Neill revealed: "At this minute Fabian has not really learned the art of tackling properly.
"He leaves his body open to being injured, which I think is something we have to work on.
"He certainly leaves a foot in and is competitive, no question about that, and he's not shy of putting a challenge in for one so slight.
"I want to keep that attitude and occasionally to curb it because you are liable to get booked once or twice. In fact, he does occasionally need to be reminded that I don't need him to put four of my own boys in the treatment room in training just to prove that he can tackle, he can save that for match day!
"So while I don't want to knock any aggression out of him, we don't want him to be going into challenges in a manner which is almost dangerous to himself.
"His body is opened up to injuries, and the slightest turn or twist he could do himself some damage.
"I'm talking about the technique of challenging. He will go in without thinking of safety to himself."