The 17 year-old has had an outstanding first-half of the season with Bayern.
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Now with Vancouver Whitecaps, Muller told Süddeutsche Zeitung: "His shooting technique is truly extraordinary. He has an excellent finish, a crucial skill in modern football.
"The question that will decide his future is how often he finds himself in those situations: whether he'll be able to create them himself or whether he'll need the support of the team."
Muller also likened Karl to his former Bayern teammate Lukas Podolski.
He added, "When 'Poldi' had the ball in the right space on his left foot, he was explosive. However, compared to the world's top players, he simply wasn't able to find the right conditions to shoot with the necessary frequency."