Heskey and Milner had to be substituted during the 1-1 home draw with revitalised Sunderland another dent to Villa's hopes of securing a Champions League spot. The weekend encounter at Stamford Bridge will be Villa's fifth game in 15 days and O'Neill may be forced to make changes after keeping faith with the same group of players in recent weeks.
O'Neill said: "Gabby is improving daily but I don't know if he will be ready for Saturday. He might be and we'll see where we are.
"Emile came off with an Achilles problem. James did well to play for as long as he did in the game. He was sore beforehand but, psychologically, I wanted him on the field.
"He was prepared to play. He wasn't going to exacerbate things greatly other than playing through a bit of pain which he's done.
"I'm delighted with him but his Achilles is very sore.
"They're all going for scans, Richard Dunne also. It's down to the number of games we've played. I think overall we've been really lucky with injuries but the games are definitely taking their toll."
O'Neill believes the race for fourth spot is wide open, especially with Manchester City losing at home to Everton.
He said: "We were disappointed in the dressing room but the Man City result keeps us still there and it is going to be really tight.
"It is still alive, absolutely. Tottenham will be delighted at the two of us dropping points while getting through to the FA Cup semi-finals.
"But we all have tough fixtures, none tougher than us for Saturday, but there will be twists and turns, as I said a few weeks ago, like Manchester City getting beat by Everton. It's so tight."