French legend Zinedine Zidane says Anthony Martial must be mentally tough to succeed at Manchester United.
Martial 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."