Young striker Obi only finalised his move from Arsenal's Academy in November but made such an impact at youth level that he was introduced for a senior debut in the away game at Tottenham Hotspur last February.
The 17-year-old scored his first senior goals in the 3-1 post-season friendly win against Hong Kong, China and performed well for Denmark's Under-20s at the Maurice Revello Tournament over the summer, which meant he only came back to Carrington on Thursday.
After previously wearing no.56, he will now be no.32 for the campaign ahead, which begins in earnest with a friendly against Leeds United, in Stockholm, on Saturday 19 July.