The former QPR youngster signed for Liverpool at the age of 15 and earned a call-up to the England World Cup squad on the back of his performances for the Merseyside outfit last season.
The 19-year-old was handed his first international start in England's 2-1 loss to Italy on Saturday. He impressed in a central role and Gallen, who coached Sterling for two years before he moved North to join the Reds, told The Telegraph that the winger can become one of the world's best players.
He said: "Raheem's incredible rise will have taken nobody at Queens Park Rangers by surprise and he undoubtedly has the potential to become one of the best players in the world.
"It was a very special moment watching him produce the performance he did against Italy and I am fully expecting him to play a key role if England are to beat Uruguay on Thursday night.
"It is fascinating to watch how his career has developed and how he is now being seen as one of the star men for England."
Sterling said yesterday that he is happy to play anywhere for the national team.