The 17 year-old fullback signed for United on transfer deadline day.
"We wanted him to stay," insisted Rizzo.
"We told him it was too soon for him to move abroad.
"But we couldn't do anything. There was no changing his mind."
United are paying an initial £700,000 for Janko, with a set fee due FCZ for every first team game he plays up to 50.
Janko trialled with United earlier this summer after being spotted by scouts at the Blue Stars FIFA Youth Cup.
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