Booy admits their former protege has improved remarkably since his summer switch to Sporting.
"It was I who advised him to choose Sporting, precisely because the Portuguese league would require more of him physically," said Booy.
"Ricky has the talent, that's why I signed him from Vitesse, but he needs to go step by step.
"The great thing about him is he gives everything 100 per cent. At Utrecht, he would show as much application in the gym as he did on the training pitch.
"He is now becoming a more powerful player."