The Black Cats have previously stated that the 25-year-old will not leave the club, but the Potters have submitted an improved deal for the player.
Poyet has admitted that he does not know what will happen in the remaining hours of the transfer window.
"We were expecting not to do any business today except if there was an incredible opportunity either way, which always appears for different reasons," he told the Northern Echo.
"From the beginning of the morning, we started to deal with the Lee Cattermole situation. I am late because in the last few hours we have got to a really impressive point.
"The offer and the situation is not easy. Nothing is finalised. It is still ongoing."
Cattermole has made 19 appearances for Sunderland this season.