The Red Devils are eager to strengthen their squad, especially with younger players.
Per The Mail, the 20-year-old Guerra has been watched by United scouts in every game so far this season.
The attacker, who has three goals and one assist so far in La Liga, has an £87 million release clause.
Miguel Angel Corona, Valencia's sporting director, stated: “We are fully aware that we have a player with enormous potential for the future and the ability to perform at a good level now.
“That is why the club have decisively moved him up to the first team in recent months."