Ross McCormack is set for a longer lay-off than predicted after new scans revealed further problems on his injured ankle.
The striker picked up the injury away to Cardiff last weekend and will now have an operation tomorrow (Friday) before learning the full extent of the injury.
In his weekly press conference ahead of the game against Forest, Warnock said:
"The surgeon has had another look at the scans and he is concerned. We thought it was a fragment of bone, but it could be a little bit bigger than we thought so the prognosis is it could be another four weeks or so but we won't know until he goes in and operates, and we'll know the decision then.
"The worst scenario could be the best part of Christmas which isn't good for us. He is a player you can't replace. We wish him all the best and hope he's back as quick as possible."
The gaffer faces further injury worries with the news that Luciano Becchio and Adam Drury are doubts for Saturday's clash with Forest. Drury like McCormack, picked up an injury against Cardiff while Becchio fell awkwardly against Hull and picked up an ankle injury.
Warnock is giving the pair as long as possible to prove their fitness, but is hopeful they'll both be back. He said:
"Hopefully they'll both be back. Luciano will do some light training. We took him off to make sure it wasn't anything long term. Putting ice on it straight away definitely helped.
"Adam will do some light training as well, but we'll leave it as long as possible. Even if we leave it as late as Saturday morning and have late fitness tests for both."