The talented young forward has returned to the Premier League heavyweights from Brazilian outfit Ituano.
He featured for a United youth XI against Lincoln on Wednesday.
The 16-year-old made a second-half appearance as United won 2-0 at at Sincil Bank.
Martinelli has already had one trial at United - and the club are keen to take another look at him.
Millen Baars, Aliou Badara Traore, Largie Ramazani and Theo Richardson have all joined United’s academy this summer.
If Martinelli continues to impress, he could also been snapped up.
United have spent big in recent weeks as they look to bolster their first team.
Romelu Lukaku was snapped up from Everton for around £75m.
Victor Lindelof has also been drafted in from Benfica for £31m.
Mourinho has been heavily linked with Inter Milan winger Ivan Perisic.
However, reports earlier today suggested a sensational swoop for Real Madrid star Gareth Bale could be on the cards.
The Welshman's future at the Bernabeu is in doubt as Real close in on Kylian Mbappe.
