The Red Devils made a surprise exit from the Champions League earlier this month following a 2-1 aggregate loss to Sevilla.
Manchester United have scored 28 goals less than Premier League leaders Manchester City in the top flight this term.
Mourinho has been criticised for his perceived negative tactics throughout the 2017-18 Premier League season.
The Red Devils have signed world class talents such as Paul Pogba, Henrikh Mkhitaryan and Alexis Sanchez over the past 24 months.
Manchester United have been linked with a potential swoop to sign Real Madrid winger Gareth Bale in recent weeks.
However, former Manchester United defender Parker believes some world class talents will be reluctant to play under Mourinho.
“Manchester United could go and sign the best player in the world but is that player going to be allowed to play and show why he’s the best player in the world?” said Parker, speaking exclusively to 888sport.
“No matter how much is spent will those players be allowed to show what their best strengths are? That’s the big problem.
“We see this all the time and these players come and it doesn’t work out because they’re no longer the players they were at their former clubs. That comes down to Mourinho’s style of football.”
Manchester United finished in second place in the Premier League table and two points ahead of Liverpool FC.
The Red Devils are in the FA Cup semi-finals but will have to get past London club Tottenham Hotspur to reach the Wembley final.
Read the full 888sport interview with Paul Parker, including the player Paul Pogba should be aspiring to in the Premier League
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