Man Utd continue to scout Romelu Lukaku despite the offer of an improved deal on the table for the Everton striker, SiaS sources say.
The Red Devils maintained a discreet distance this summer after his father revealed their original interest.
Lukaku's former club Chelsea are reportedly still willing to part with £65m to take the player back to Stamford Bridge.
United, however, had spies in the crowd to see the 23-year-old score in last Friday's 1-1 draw at home to Crystal Palace - his fifth goal of the campaign so far.
It's understood the Merseyside club were aware of their presence and it will only speed up the current efforts to get him to sign a new five-year contract worth £100,000 a week.
Everton's concern that he could end up at United is further heightened due to the player's agent, Mino Raiola, who was responsible for taking Zlatan Ibrahimovic, Henrikh Mkhitaryan and Paul Pogba to Old Trafford this summer.
United sources have confirmed boss Jose Mourinho is on the look out for fresh striker talent, which will only add further uncertainty to the future of Wayne Rooney.
Mourinho has forged a decent relationship to Raiola and he will certainly know how the land lies on a player he allowed to permanently join the Toffees for £28m back in 2014.