"For a player to be selected for his country, normally he must be first choice at his club," Mourinho said.
But he was keen to stress a player needed regular football to be certain of a starting place in Brazil 2014.
"He must play very regularly with the club and I think that's the big motivation if they are thinking about the World Cup," Mourinho added.
"So any player that wants to go to the World Cup, if they are a second choice in their club, they are in trouble... they are in trouble.
"So I think this is a crucial year for that and probably many national team coaches are worried because they want their players to play regularly in their teams."