The Red Devils brought the Portuguese on board soon after sacking Louis van Gaal two years into his tenure, backing him with large funds to transform the squad but only managing a sixth-placed finish in the Premier League.
Souness believes that United made the wrong call when turning to Mourinho, however, and says that rivals Manchester City's coup to land Pep Guardiola rather forced them into action sooner than they would have liked.
"I didn't think Mourinho was right for United and I said that from day one," he told Yahoo! Sport. "Because Guardiola was coming in at Manchester City, the people making decisions at Old Trafford felt compelled to go and get a big name manager and they ended up with Jose.
"He has taken on a very difficult job at United and struggled to find his find his best eleven last season, if he ever found it. A big concern for me that their best player for most of last season was a 35-year-old in Zlatan Ibrahimovic, with 35-year-old Michael Carrick also one of the top performers.
"Mourinho is used to winning league titles and finishing where he did last season in the Premier League will not be acceptable, but it won't be easy to put it right."
Mourinho ended his first campaign at Old Trafford with two major honours, lifting the League Cup and Europa League, while also prevailing in the Community Shield.