Memphis was recalled for victory at Watford on Saturday, scoring in the 2-1 win.
De Boer told De Telegraaf: "Memphis has the potential to succeed at the very top, but there are still some things to look at.
"Can he withstand a setback? Is he prepared to roll up his sleeves when it doesn't come so easy? Is he self-critical?"
The former Ajax, Barcelona and Rangers star added: "Cristiano Ronaldo did not directly reach the summit, when he came to Manchester United. I remember that there was much doubt about whether he would make it.
"Ronaldo was primarily criticized for his weird antics and mocked as a circus act. But look where he is now."
