Manchester United midfielder Shinji Kagawa admits he has been "very irritated" by his own form this season.

United had high hopes for Kagawa when they signed him from Borussia Dortmund two years ago, but the Japanese has struggled to hold down a place in the starting XI.

Kagawa showed glimpses of his potential last term - like when he scored a stunning hat-trick against Norwich - but he is yet to find the net this season.

The 24-year-old has completed just two Premier League games and he concedes he must improve.

"I still haven't scored any goals this season so I've felt very irritated," Kagawa told Sky PerfecTV. "This is the first time in my career that I haven't been able to produce so it's disappointing."

Juan Mata's arrival at Old Trafford is likely to decrease Kagawa's chances of first-team football, and Kagawa's confidence appears to be very low.