The 18-year-old striker has impressed watching scouts from a whole host of Premier League clubs, including United, City, Chelsea and Liverpool. And the player's club, Sporting Lisbon, already value Mane at around £8m, according to the report.
The young forward has been a leading light in hugely talented young Sporting Lisbon youth team. The Portuguese outfit won the NexLions Cup in Singapore at the start of December - during the tournament Lisbon beat Liverpool on route to the final.
Scouts from United and City watched on as Mane impressed, but the Premier League title rivals will likely face competition from Chelsea and Liverpool, should they move for the teenage starlet.
United have a strong track record when it comes to signing highly-regarded young attackers from Sporting. The Old Trafford club bought Cristiano Ronaldo and Nani from the Portuguese club, and both have gone on to become Portugal internationals - in Ronaldo's case to become national team captain.
But City, with an new and avowed focus on developing young players, have been linked with a number of the world's best young players, and would be keen to rival United for the signature of Mane, should it become clear that Sporting would be willing to sell. At this stage, however, the Premier League giants are only tracking the young forward.