The 21-year-old defender earned plaudits for his exploits at Real Valladolid last season.
Saints finally completed a deal for the Ghana centre-back this week, and Hasenhuttl couldn't be happier.
"This is an important signing for us," said the Austrian."Mohammed is a player who fits our profile well.
"He is young and has a great amount of potential, but he is also someone who has the qualities to come in and help the team as soon as he is up to speed with our way of playing.
"I like what I have seen of him so far in his games with Valladolid. He is strong defensively, he is calm with the ball – something that is important in our team – and he has good speed too.
"I think he will fit in well with us and that this will be a good place for him to develop his game further. We have some good centre-backs here and this will be another strong option for us."