Although the 25-year-old winger has shown glimpses of form since then, O'Neill believes there is more to come as Downing regains full fitness.
"I have been very, very pleased with him," O'Neill told the club's official website.
"I still think, even though he has played a number of games in quick succession with no respite, that he is not fully wound up yet.
"But he's given everything. When he is fully wound up, he will be really excellent for us."