The 25-year-old has been in impressive form for his Portuguese club during the 2021-22 campaign, scoring 16 times and registering six assists in 28 appearances in all competitions.
Diaz was again on the scoresheet against Famalicao on Sunday, netting his side's second in what proved to be a 3-1 home success.
According to O Jogo, Man United had representatives at the game to watch the Colombia international, who continues to linked with a move away from his current club.
Diaz has scored 41 goals and registered 19 assists in 125 appearances for Porto since making the move from Junior FC in 2019.
Liverpool have also been credited with an interest in the South American, and a report earlier this month claimed that the Reds were prepared to pay £60m to sign him before the end of the January transfer window. body check tags ::