Brazil international Fred and Serbian star Matic both arrived at Old Trafford while Mourinho was in charge of the Red Devils.
Mourinho had worked with Matic at Stamford Bridge and signed the midfielder from Chelsea, while Fred joined from Shakhtar Donetsk in a deal costing around £47million.
Souness believes the time has come for Manchester United to move on from Fred and Matic and described them as a pair of ‘Steady Eddies’.
He told The Times: ‘Scott McTominay is an honest player and, at 24, is only going to get better, but do they win the Premier League with Nemanja Matic and Fred, who they paid £40m and £47m respectively for under Jose Mourinho?
‘They are Steady Eddies who will dog it out away from home and protect your back four, but get them on the front foot and there’s no cuteness in their play.
‘The midfield is the creative arm of the team and other than Bruno Fernandes, United need someone with cuter passing, who is a better all-round player than what they have now.’
Manchester United are considering signing a new centre back ahead of this summer’s transfer window, with the current pairing of Harry Maguire and Victor Lindelof still displaying vulnerability.
But Souness does not believe signing a new defender should be Man United’s ‘priority’ at the end of the season.
‘Much is made of their back four, but you can’t buy 11 new players,’ he added. ‘You strengthen where you are weakest in the squad and, depending on what becomes available, I don’t see the back four as the priority this summer.
‘Solskjaer is doing a good job but finishing second with this squad would not have happened in recent seasons – so nobody at United should be fooled into thinking that they are ready to challenge for first next season as things stand.’
Manchester United continue their quest to secure a top-four finish on Sunday afternoon when they face Premier League leaders Manchester City.