The 17-year-old, who recently signed a lucrative professional contract with the Red Devils, made his first-team debut for Ole Gunnar Solskjaer's side as a late substitute against Newcastle United on Saturday.
Shoretire has been with United since 2014, but according to the Daily Mail, the youngster had been with City's academy before being released at the age of 10.
The report claims that the Citizens made the decision after discovering that the youngster had undergone a brief trial with Barca during a family holiday.
Shoretire had been in impressive form for United's Under-23s outfit in Premier League 2, scoring six times and registering four assists in 14 appearances before travelling with the squad to Turin for the Europa League last-32 first leg against Real Sociedad.
The teenager was an unused substitute in his team's 4-0 European win but was handed a late appearance against his former club Newcastle in the Premier League.