The 31-year-old trained with the club this week and his signing was confirmed ahead of Saturday's 4-0 win over Shrewsbury in the Capital One Cup at Elland Road.
Warnock said after the win over the Shrews: "He trained with us on Thursday and Friday and I will make a decision on him before next weekend because if I sign him (permanently) I will want him to be involved against Wolves.
"We have no money at the moment so he won't be retiring on the wages we can offer, but he is available on a Bosman transfer and he has genuine quality. He has always been a wild card and we have had our moments in the past, but I met him at a function a few weeks ago, I told him what I disliked about him and how I felt he could improve and it was a good conversation.
"I also spoke to a few people who know him well and they told me how good he is in the dressing room."