The Black Cats are without their midfield general for a tough run including ties with both Manchester clubs, Chelsea and Tottenham, with Steven Fletcher also a doubt for Saturday
The midfielder was forced off during the defeat to West Brom in November, before managing just six minutes of the 0-0 with QPR three days later.
The 24-year-old is suffering from medial ligament damage to his knee and O'Neill has confirmed that a prolonged spell out of action is in order - something the manger had suspected anyway.
"He'll probably be about another two months," the northern Irishman told reporters. "I knew the news wouldn't be too good. It could have been worse but obviously it could have been better."
|BLACK CATS ON THE RISE?|
|22/1||Sunderland are 22/1 with bet365 to beat Chelsea 2-0 this weekend|
"I’ll make a late decision on Fletcher," the 60-year-old added. "He’s been down to see the specialist, who was relatively pleased with him.
“He’s had an injection and we will now have to see how that works. Steven didn’t train yesterday but hopefully he will today and we’ll see how he gets on.
“But the two games in four days is something we might have to consider.”