A target for Tottenham, the Denmark midfielder has just over a year to run on his current deal.
Hasenhuttl, who penned a new four-year deal with Saints yesterday, told PA Sport: "He is a fantastic focused team player and he has his goals.
"We know what kind of club we are and what we want to be in the future. When a player performs well and gets an offer from a big club, and that is a realistic - and not a silly - offer that we can accept, then we can agree on everything.
"But he has one year more of his contract left, and if the offer is not good enough, he will stay here one more year. We are not forced to sell him."