It has been reported that French legend Zinedine Zidane says Anthony Martial must be mentally tough to succeed at Manchester United.
The French international left AS Monaco for United in August for a fee rising to €80 million.
Real Madrid Castilla coach Zizou told Canal+: “It’s good that there are players who are emerging, but he’ll have to prove he is worth €80m.
“This is what is harder, especially at 19 years of age.
“When I made my transfer for €75m, I was 29 years old, but what’s important is everything that surrounds you. You have to be strong with such a transfer.”