Ferguson retired from football in May 2013, with Scottish compatriot David Moyes coming in as his successor.
Moyes - now at Real Sociedad - was sacked in April last year towards the end of a season that saw United miss out on European qualification as they finished a disappointing seventh in the Premier League.
Former Barcelona, Bayern Munich, Ajax and Netherlands boss Van Gaal has enjoyed better fortune in his maiden season at Old Trafford, with United sitting third in the Premier League and progressing to the fifth round of the FA Cup.
However, agent Mendes - who represents United duo Radamel Falcao and Angel di Maria as well as the likes of Cristiano Ronaldo and Diego Costa - insists they would be doing much better with Ferguson still at the helm.
"It's completely impossible to find someone like Sir Alex Ferguson because he's a god, he's a genius and to find a genius is very difficult," Mendes told BBC Sport.
"You have one in England - Jose Mourinho. Having Sir Alex Ferguson ensures at this moment you have 12 or 15 points more.
"He's finished his career, but he's someone that's the difference."
