Cahill made the bench at Liverpool for the first time since the opening day of the season against the Reds.
"I understand Gary's position in that he wants to play, but I have to look after events here," said O'Neill.
"Having done so well at Sheffield United on loan, he obviously doesn't want to stall his career.
"By the same token, I'm not in any rush and I will continue to monitor Gary's situation right up until transfer deadline.
"It also depends on Olof Mellberg's situation and I need to assess his frame of mind if he has decided to move on."