Ruud Van Nistelrooy's penalty handed United a 1-0 win at the Lane on Saturday and condemned us to a first defeat in eight matches, seven in the Premiership and one Carling Cup tie.
Disappointed afterwards, Simon was quick to turn his attention to our trip to an in-form Everton this weekend.
"You are not going to stay undefeated all of the time and it's no disgrace losing to Manchester United," said Simon.
"We've got to bounce back straight away. We play Everton on Saturday, they are right up there after their start but we'll be looking to get a result of some kind at Goodison."
The midfielder started on the bench as Jacques Santini changed tactics against United but was soon into the action as Fredi Kanoute departed with a hamstring problem.
"I thought it was difficult in the first half," he added. "We were disappointed at half-time but we changed things around, got at them more and created a couple of half-chances.
"The second half was a lot more positive after the first half where I felt we let them play too easily.
"Perhaps we gave them a bit too much respect in the first half. We came in at half-time and the manager told us to believe on ourselves a bit more. We did that.
"All in all, the first half was disappointing but we can take positives out of the second half where we went at them, got amongst it and made it more of a game."
Simon Davies insists the disappointment the lads felt against United will quickly turn into determination to bounce back at Everton.