The Perth side have failed to find the net in their last four league games, while opponents Dundee United had been bagging goals galore before Boxing Day's surprise 4-1 defeat away to St Mirren.
Now, having run champions Celtic close in a 1-0 loss at McDiarmid Park, the Perth goalkeeper admits a derby win would carry increased significance.
St Johnstone don't want to fall any further off the pace and the Northern Ireland international admitted: "There are lots of reasons we want to win this game.
"It's a bit of a derby so it is important to the fans. But it is important to us to start getting points again and scoring goals.
"It's obviously going to be a tough game. Up until now they have been scoring loads of goals.
"I don't know why but we were sluggish in the first half of our games against Hibs and Celtic. So we have to make sure we start well. We have to make sure we are ready.
"I hope we take a lot out of our second-half performance against Celtic. We only started to think about United after the Celtic game was out of the way.
"I suppose [Dundee United's] result against St Mirren does show they can be beaten. It was a surprising scoreline.
"We haven't scored in a few games now and it would be good to start pegging back teams above us in the top six - and get out of this rut and start winning games like we were doing before. It would be nice to do it in this game."
While United have been attracting kudos for the precocious talents of their youngsters, Mannus is adamant Saints have their own box-office stars.
He said: "People in Perth should be wanting to watch guys like Stevie May, Nigel Hasselbaink and David Wotherspoon.
"We have a bit of a mixture with others who are just going to be consistent and do their job, do the basics right - players like Dave Mackay and Frazer Wright.
"I think fans should want to see those sort of players."