The Sun claim the player - currently on loan at Vitoria Guimaraes - could be on his way to Old Trafford as super-agent Jorge Mendes represents him.
Jose Mourinho is also a client of the Portuguese as well as Real Madrid superstar Cristiano Ronaldo.
And Mendes is said to have alerted United over Mensah's form.
The Ghana international was in action at the weekend in the 1-1 draw against Belenenses in the Primera Liga.
United, Monaco and Sevilla have sent scouts to watch the 21-year-old as they consider moves for the starlet.
Mensah was previously linked with the Red Devils in the summer of 2015 but opted to join Spanish giants Atletico instead.
The midfielder spent last season on loan at Getafe but failed to impress.
Mensah has a six-year contract with Diego Simeone's side which runs to the summer of 2021.
United have previous dealings with Guimaraes, signing flop Bebe for £7.4m back in 2010.
