The former Borussia Dortmund playmaker has struggled to establish himself at Old Trafford since his £12m move in summer 2012.
He told United's official website: "I think I can still improve my strength. I'm not the tallest or biggest player, so I have to move my body more.
"Smaller players in the Premier League are very good at using their bodies (to get past the opposition) and that's what I will continue to do.
"I realise the Premier League is a very tough league and I know I have to be stronger."
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