The £2.7m defender has become a regular in Souness's squad, despite being tipped as one for the future on his arrival.
And Souness admits that the Sweden U-21 skipper has solved a long-running left-sided problem in his time at Ewood Park.
He said: "We are very happy with Nissa. He is still only 21 and he came here for quite a big fee, but he has adapted very well.
"We spent a long time looking for a defender who was comfortable on the left-hand side and Nissa has done really well.
"He is very keen to learn and if he continues doing as he is, then he will be a super player."