Barkley has won just three caps for the Three Lions, but Jagielka has backed him to make a big impact in Brazil.
"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% 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."
England's World Cup campaign begins against Italy in Manaus on June 14.