Nigel Clough says positive steps are taking place with regards to Paddy McCarthy returning for a second loan spell at United.
McCarthy played five games, scoring one goal, ahead of his loan spell expiring after Saturday's game against Barnsley.
"We are hoping to get Paddy back, but it will be after Saturday, Crystal Palace don't want us to play him in the FA Cup," Nigel Clough said on Wednesday.
"Paddy has been a brilliant influence in our camp, he brings balance, leadership and experience - we look a lot more solid when he is around and he has formed a solid partnership with Jay McEveley.
"It's agreed in principle at the moment, we have to get the paperwork sorted but we are once again very grateful for Neil Warnock's help, if we can secure Paddy's signature until the end of December we will be delighted."
Palace defender McCarthy, 31, signed a month's loan at Bramall Lane in October, scoring on his home debut against Leyton Orient.