The midfielder believes he could have offered more to Sir Alex Ferguson's side.
He said: "I was looking to produce even more results. I was injured and I felt things didn't go well easily.
"I had a lot of conflict, frustration and worries. In that sense, I think it has been a profound year.
"Honestly, I haven't contributed as much as I did at Dortmund.
"I was even injured. I still don't quite have that feeling of accomplishment."