Forest lodged a bid of £14m for Engels on the weekend, which the Hoops instantly rejected.
Celtic manager Martin O'Neill said after their win against Falkirk: "Honestly, I am telling you, I didn’t know anything about it.
"If they have done it, then they must have done it during the game. And I would want it rebuffed, unless it was £100m.
"Then I think the board would step in there. No, absolutely, I would rather keep him."
Celtic are expecting a new offer from Forest for Engels before the winter market deadline falls.