Saville put in an improved display in Saturday's thrilling 4-3 win at Tranmere Rovers as the Bees got their promotion push back on track.
However, Rosler wants the youngster to stick to his guns when the going gets tough.
"Players have to learn quickly," said the German boss.
"I'm happy with George Saville. He hasn't had his best two games but his overall play was much better.
"But he has to learn how to get wins over the line. In the last 10 minutes there were two goals from his side."
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