The 23-year-old joined United from German champions Borussia Dortmund in the off-season and has failed to reach the heights he did during his tenure at Signal Iduna Park.
The Japan International, who reportedly cost the Red Devils £17million, was substituted early in the second half in the dramatic 1-1 draw with Spurs at the weekend after another sub-par showing in a red shirt.
"I'm really feeling that I need to perform better," he told the Daily Mirror. "I'm not satisfied with what I have done here at all.
"It's a big and really famous club in the world and I need to overcome lots of pressures and be mentally stronger."
"Unfortunately I got injured, but I have to be better and grow as a player a person, in terms of contributing to team's wins and succeeding here."