Saints are enduring a torrid time in the league but will have opportunity to switch their focus to their local rivals.
"The cup for us is to have a chance to win a trophy and I think this is why we took it very seriously from the beginning," said Hasenhuttl.
"We didn't pick our best team against Wolves and Arsenal to now say it's not important for us. It is very important.
"We said when we were in a better position in the league we could also concentrate on the cup, with the risk we have then a few more injuries maybe.
"But we took this risk and because of having a game like we have on Saturday. A game where we have a chance to get to the semi-final of the most traditional competition in English football.
"I think it is for us, for the players, for the staff, for the fans, for everybody in this club, maybe the most important game of this year."
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