Wolves managed to pick up a 2-1 win against Manchester City last Saturday, but lost to United in the Carling Cup last month.
“The result on Saturday helped do a whole lot for self-belief and confidence,” McCarthy told the Express & Star.
“No matter what you do if you keep getting beat you start to wonder where the results are going to come from.
“We’ve been playing well without getting the results and we needed to keep it going.
“It’s alright upsetting them, you’ve got to really upset them and take something off them – and wouldn’t they be nice back-to-back results?
“Seriously, why shouldn’t we get any points out of them?
“They’ll have had us watched on Saturday and will know it won’t be easy. We upset Tottenham for 75 minutes and Chelsea for 80 minutes.
“We did it on Saturday against City and the previous Tuesday against United.”