The Welsh outfit will be missing international stars George North, Rhys Priestland and Jonathan Davies because of the Six Nations but Manu has urged those who will step in to the breach to stand up and be counted.
The victor at Franklin's Gardens will face English giants Leicester, who beat Bath 17-16 on Friday, in the final on March 18 and Manu is determined to fly the flag for the RaboDirect PRO12.
"There are no second prizes and there's no next week in knockout rugby," said Fiji international Manu speaking to the Western Mail.
"That is going to be a huge emphasis. This is the be all and end all as far as this competition is concerned.
"Making the semi-final is a great achievement for this squad, but this is an opportunity for us and we need to push forward.
"The depth of the squad has been brilliant and the young guys coming through may have been new faces to some people, but they have been in the academy system here for a while.
"People like George North and Jonathan Davies have done that and others are coming through now as well.
"It is probably the strongest squad since I have been here. In the past we have been missing one or two key players which had a detrimental effect to our performance. But boys have done an ample job of stepping up."
Manu and co. have plenty to be positive about having already tasted victory in Northampton once this year with a 28-23 win in the Heineken Cup pool stage in November.
And after back-to-back wins in the RaboDirect PRO12 Manu has no doubts Scarlets can go all the way in what will be their first semi-final of a cup competition since they lost to an Andy Goode-inspired Leicester Tigers in the Heineken Cup in 2007
Manu added: "You look at our recent results in the RaboDirect PRO12 and we take positives from that, they are missing players like us, but we are under no illusions how hard it is going to be.
"A lot of their forwards are still there. They are fourth in the Premiership and were finalists in the Heineken Cup last year, they are going to be a tough proposition.
"We have been watching a lot of their footage and their forward play is a key area for them.
"They also play a good style and good brand of rugby when opportunities are on they look to score out wide.
"We have to do our homework on them but not hold back on our game, we still have to go out and get a result, but we know it is going to take our best 80-minute performance this season."
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