McCarthy said Tau should continue to work hard, saying it will be up to the coach to decide whether or not he's ready to face Zinedine Zidane's charges at the Santiago Bernabeu.
"The kid is doing what he is supposed to do. You can only work hard but at the end of the day, it is down to the management and coach to select you but I watch them. I do switch on to watch Club Brugge and I like to see how Percy Tau is progressing," McCarthy told the media.
"They've got a strong team and he must just keep working [hard]. The season is long and good or bad players. You still get injuries and when the call comes you have to take it."
McCarthy, who is still the only South African to have won the Champions League, said facing Real Madrid should be like a normal game for the 25-year-old left-footer.
"Real Madrid should be like a normal league game for Percy - do exactly the same because the minute you start adjusting your game, you are putting too much pressure on yourself. It is a bigger team and a different occasion," added McCarthy.
"Just do what you do and prepare how you need to and if you can do a little extra, do it but he shouldn’t let the occasion play him because he plays at the Bernabeu. People want to stand and look at where Ronaldo played. He must just play his own game. He is a good quality player to not be fazed by that."