The 24-year-old midfielder is due to see his specialist within days after having an injection in his injured knee earlier this month in a bid to solve a long-running problem.
However, while hoping that surgery will not be required, O'Neill is facing up to the prospect of having to do with his captain for most, if not all, of the remaining eight games of the Barclays Premier League season.
He told the Sunderland Echo: "Lee has a few more days to go after having had the injection.
"The specialist told him not to do a great deal and while he is obviously working with the physiotherapist, he is not doing anything that will risk damaging his knee.
"We hope for the best but, to me, if he comes back this season, then that would be a bonus - and an unexpected bonus.
"We may have to cope without him - and even if he does successfully come back from the injury, he will still be nowhere near match fit."
Cattermole has managed only 14 appearances to date this season and has had to sit back and watch helplessly as the Black Cats' winless run has extended to seven games, in the process plunging them into relegation trouble.