James Collins, eight, spent £93.20 of his birthday money on a new Man U kit, paying to have the name of his idol, Januzaj, and his number, 44, printed on the strip.
But a month after splashing the cash, Januzaj switched from 44 to number 11, leaving James's kit apparently 'ruined'.
Januzaji has played under 44 for several years, but switched to number 11 after it was left vacant by Ryan Giggs' departure from the club.
Manchester United did not inform fans about the change until August 17 – a month after the new home shirt had been released for sale.
James is now refusing to wear his new branded set, because he is worried about being teased.
His mum Cheryl, from Longsight, Greater Manchester, asked the Manchester United megastore for a new shirt, but was told any printing is done 'at the shirt owner's risk'.
Cheryl said: "He's United mad. He has spent all summer at football camp and it's all he ever talks about.
"He couldn't wait to get the new shirt and kept asking when I was ordering it. He'd only worn it two or three times when we found out he [Januzaj] had changed number.
"We emailed and asked them if he could change it and they said no because there was a disclaimer with all printing. A few weeks ago the club replaced a shirt where someone had spelt Rooney wrong. That was their error.
"Yet we have done nothing wrong and have been left out of pocket. If we had any idea it was going to change I would have made him wait rather than see him disappointed.
"James wanted the third kit as well but I've put a stop to that now. I'm not going to let him spend money on a shirt that could be out of date straight away."
A spokesman for the club said that notices were prominently displayed throughout the store making clear that squad numbers were not final until they are registered with the Premier League.
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