Ellis, who appointed O'Neill as manager four years ago, insists "the club is bigger than any one man" after O'Neill resigned yesterday because of a dispute over transfer policy.
Ellis believes the timing of O'Neill's decision, just five days before the Barclays Premier League campaign, is far from ideal.
He told Sky Sports News: "I was not entirely surprised by Martin's resignation although I was disappointed by the timing of it.
"I'm saddened. I think the supporters will be saddened - on the whole. "Martin has done a very good job for Villa and I'm glad I appointed him.
"It is just a pity it has been done at this particular time."
Source: PA
Source: PA