The 24-year-old has completed just two Barclays 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.
"I pressure myself to score in every game but some days it feels like I just can't win. I know I have to overcome that," he added.
"Actually, if I can't it really shows that I haven't grown as a player. So because of that I have to really focus on the second half of the season and overcome that mental block."
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