Michael Laudrup's side stunned Valencia 3-0 at the Mestalla Stadium in the Europa League on Thursday.
Routledge said: "Playing at massive stadiums against massive clubs like Valencia and winning 3-0 will only ever give you a big confidence going into the next game which is against the club where it all started for me.
"The start we had was always going to be tough knowing we had to juggle the Europa League qualifiers and those first two big fixtures (against Manchester United and Tottenham). But we got a great result against West Brom and put in a good performance against Liverpool and it was another great performance against Valencia which shows we're growing as a squad as well.
"So if we put what we did in those three games together I'm sure we could come away with a good result on Sunday."