The youngster has been a revelation for the Baggies this season.
Southgate told the Mirror: "It's great when you see a player you are working with score the winning goal at Old Trafford. He's in a really good place at the moment.
"I went to Colombia with the under-20s, he was playing up out of his age group. Lovely kid, can finish, terrific pace, great work ethic and when we were working with him last month we were thinking: 'where's the catch?'
"He's a very humble boy. I'm sure there will be peaks and troughs as the season goes because a lot of the players are playing a lot of football.
"But over the past 18 months, he's really come on. He wasn't playing (for West Brom) a month ago so it's down to us!"
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