The Norwegian international has scored just once since the new boss took over club last season and has found himself on the substitutes' bench behind transfer-listed Welsh international Nathan Blake in the order up-front during pre-season.
Souness said: "I've been disappointed in what I've seen of Ostenstad since I came to Blackburn, but if that's a criticism then it's an honest one.
"What he did at Southampton he hasn't done at Blackburn."
Ostenstad came to English football when he joined The Saints from Norwegian side Viking Stavanger for £800,000 in October 1996, and had a good spell before his goal-scoring production dropped.
He then moved on to Blackburn last season in a swap deal with Kevin Davies going the other way, but has failed to perform in the First Division.
Meanwhile, Souness has been linked with a move for Wimbledon striker John Hartson but has denied reports that he is looking to sign Derby County defender Rory Delap.
