Buttner believes he is continuing to improve since his move to United from Vitesse Arnhem.
"I feel like every game I play makes me stronger," he told the Manchester Evening News. "I spend a lot of time in the gym here after training as well.
"In Holland I didn't do that. That has really helped me. I think I have improved a lot in the last year and I think there is a big difference in the player you saw one year ago to the player you see now."
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