Celtic manager Neil Lennon last week told of his keenness to turn that move into a permanent one.
A fee of £2m has now been agreed between the two clubs, and the proposed move now hinges on whether Forster can agree personal terms before the end of this month’s transfer window.
Forster, known as 'Lurch' at Parkhead, said: "I don’t see myself going back to United.
“Celtic have first option at the end of the summer.
“To be honest, I’m happy leaving it to the summer.
“I would rather just concentrate on football, and that can all be done in the summer when we’re all off and there’s more time. We can sit down and sort it all out then.”
However, United’s willingness to let him go, and Lennon’s faith in him, could change his mind.
Neil Lennon (Celtic boss): “Good keepers are hard to find, and we believe he has the potential to go on and become a great keeper."