United reportedly sent scouts to watch the 25-year-old in action at the weekend, according to Sky Sports News, as the Premier League champions consider their transfer options for the summer.
It is understood that Siqueira is valued in the region of £10m, with Granada prepared to listen to offers at the end of the season.
"I am very happy with this news. If a good proposition arrives I am open to studying the subject," the Italian told reporters.
Barcelona and Valencia are also reportedly interested.