The Englishman, the club's top scorer this season with seven, will become a free agent in 2022.
"This is not a big surprise when I say he is very important for our game, he knows this, we know this," said Hasenhuttl ahead of Saints' FA Cup third-round clash with Shrewsbury Town.
"We have won games without him, but for the long term, we really feel it and the quality he has is definitely outstanding and definitely important for us.
"It is (difficult) for us a club to sub him or to use somebody who is at the same level as he is, so this is also why we really tried to extend his contract, this is not a surprise and not a secret
"But, as always, we are in the negotiation. We have at the moment a tricky situation for every club in the Premier League.
"I think everybody is losing a lot of money at the moment with this crisis we are going through, so we can only do what we are able to do and what is still (financially) healthy for us and that's why we are looking I think.
"Danny chose to be with us, he wants to be with the club and we know that he's an important player for us and I'm still hoping that we find an agreement and then finally he stays with us.
"But if not life will go on and this club will still exist and hopefully we find a way that we can both live with it."
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