The Zimbabwe striker told City's website: "I know Manchester United is a big debut so I am excited.
"We must go there, try hard and give as much as possible to get some points. This is a team with big quality and I hope to fit well with them.
"I am confident that playing with good players you must score goals.
"Every player wishes to play for a big club. I enjoyed my time (at Portsmouth) but in life and in football you have to move on."