The duo have both been named in Roy Hodgson's 23-man squad for Brazil, along with fellow Everton man Leighton Baines.
Barkley is one of a number of young and inexperienced players in the squad - along with the likes of Raheem Sterling, Luke Shaw and Adam Lallana - as Hodgson puts his faith in a more youthful generation of players.
And Jagielka, expected to be first-choice centre-back alongside Chelsea's Gary Cahill, believes Barkley will be key to England's chances in South America.
"Ross is a fantastic player," he told Sky Sports News. "He's very frustrating to play against because he has got a tremendous amount of ability.
"I'm not 100 per cent sure what role he will have during the World Cup but I'm sure it will be a very important one, whether he'll be starting or coming off the bench.
"He's grown not only as a player but as a person in the last 12 months. You can see the confidence of playing week in, week out has helped his game develop.
"We still must not forget he is only 20 years old and hopefully there will be a lot more World Cups and a lot more international appearances for Ross in his career.
"But no doubt we as a nation will get a little bit carried away this year in the World Cup and hopefully we'll be shouting about Ross at the end of it."
