Villa go into the game above Guus Hiddink's new side and know victory could go a long way to securing Champions League football.
O'Neill has been without Heskey, a £3.5m January capture from Wigan Athletic, since he aggravated an Achilles injury while on England duty in Spain and remains a doubt for the weekend. John Carew's goalscoring appearance against CSKA Moscow in the UEFA Cup does though provide an option to fall back on.
But of more concern would be the loss of Milner, a player who has been hitting top form over the last month and has scored a number of goals. The England Under-21 international, injured against Everton in the FA Cup, was also left out of Wednesday's UEFA Cup game in a bid to make sure he is fit to face Chelsea.
O'Neill said: "Milner has got a bit of a thigh problem and the doctor thought he would have a better chance for Saturday if he didn't play against CSKA.
"Heskey is improving and we will see how he is during the next 24 hours. I will have a better idea on Friday.''
O'Neill also revealed he would be seeking an audience with England coach Fabio Capello to ensure there is no repeat of the Heskey situation.
"I have said already that I wasn't best pleased,'' he added. "I haven't spoken to Capello yet. I will do. I think it is important for me to speak to him. He is coming to the game on Saturday.''
Villa are currently two points above Chelsea in the table, and three points behind second placed Liverpool, but despite the Blues' recent troubles O'Neill is expecting Hiddink's men to rise to the occasion.
"Obviously Guus Hiddink has got quality players at his club and it is only a few months ago that Chelsea were the only side this season to have beaten us very well,'' he added.
"Anyone coming away from the game that evening would have been surprised to have seen a change of manager by this time.
"But things can happen, so fickle is the game.''
O'Neill believes Hiddink would find it difficult to manage both Russia and Chelsea on a permanent basis.
He said: "The dual role is incredibly hard. I think there might be a reason for doing it on a short-term basis [at Chelsea].
"But overall, I think it would be very hard to combine an international management role with one in the Premier League over the fullness of time.
"I'm not sure that's what they are looking at [at Chelsea]. Eventually - when you lose a game or two - someone will accuse you of not spending enough time in one particular camp.
"I think it is exceptionally difficult. Nothing is impossible. Over a short period of time, I'm sure it could work but over a three or four-year period, I think that would be particularly difficult.''