The £17 million signing from Borussia Dortmund replaced Javier Hernandez in the dying seconds of United's pre-season curtain raiser, with Federico Macheda's first-half goal enough to secure Sir Alex Ferguson's side a 1-0 win.
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Ferguson explained that he had not planned to use Kagawa at all, but was forced to bring him into the action late on when it became clear that Hernandez needed to be replaced, although the new signing was left waiting for some time on the touchline before he could take to the pitch.
"Chicharito got some cramp and had to come off," he told the club's official site.
"I never thought the boy was going to come on. He must have been standing on the touchline for 10 minutes.
"It's amazing when there are so many stoppages in the second half, but it never seemed to go out when we wanted it to!
"He only came on because of Chicharito having cramp. I think he'll probably start on Saturday."
Young midfielder Nick Powell remains a few days behind schedule and is a doubt for United's next fixture, which is their last in South Africa before they leave for China to face Shanghai Shenhua and Didier Drogba.
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Ferguson, who, also moved to praise wantaway striker Dimitar Berbatov's performance, was content with his team's display in their opening pre-season fixture.
"We're satisfied," he added. "We always like to win our first game. It was a good work-out for us.
"In the second half, I thought AmaZulu increased their aggression and got closer to us, making it more difficult.
"In general, we showed some signs of tiredness towards the end, but we expect that in the first game of the tour. We played some good football and scored a nice goal. It was a good match."