United conceded seven goals at Elland Road for the first time in their history against the struggling Reds last night in one of the shock results of the season.
And Warnock admits there will be no let up for his side at The Den.
"If you think that's tough wait for Saturday," he said.
"I have learnt more tonight than any other game but I think I'll learn more on Saturday now.
"I think to come off the back of a result like this and go to Millwall and play against a side who will be flying to get the points will show me who wants to be here next season.
"This is the time to stand up and I think it will be interesting on Saturday."