Southgate has taken a relatively inexperienced group of players to Russia compared to previous tournaments, with Joe Hart and Jack Wilshere among the big names left out.
England face Tunisia in their opening game on Monday, with Panama and star-studded Belgium their other opponents in Group G.
But Mourinho, whose Manchester United players Jesse Lingard, Ashley Young, Phil Jones and Marcus Rashford are all in the squad, feels England have the talent to go all the way.