Berahino penned new terms with the Baggies yesterday.
Garlick said: "It was always our intention to reward Saido for the huge strides he has made over the past few months.
"To be able to reward one of our proteges, who has worked his way up through the ranks from the age of 11, is a great moment for the club.
"It's a fantastic story for us and especially for our hard-working academy staff, who have played a fundamental role in his development over the years.
"Producing one or two players who can supplement the first team is something we want to achieve every year.
"We hope this is the start of that process as we feel we have a number of other young, exciting players coming through."
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