The 29-year-old has failed to impress for United so far this season.
“I know some people haven’t been satisfied with my recent performances,” he told the club’s official website.
“It is not a hangover from the World Cup, but perhaps I was not mentally fully prepared for the new season.
“I’m not worried. You have many ups and downs as a player and I’m sure I will get better. The friendly with Japan will be the first step.
“This international break will be the turning point for me and my performances will improve when I return to United.”
