According to talkSPORT, Estupinan has emerged as a wing-back target ahead of his likely Brighton exit this summer.
It's claimed that United hold a genuine interest in the Ecuadorian international and have already held talks with his representatives.
The report adds that Brighton have not yet received a formal approach, but the Seagulls are willing to cash in after Maxim De Cuyper's £17.5million arrival from Club Brugge.
Since his 2022 arrival from Villarreal, Estupinan has made 104 appearances for Brighton, scoring five goals and registering 14 assists.
In June, he strongly suggested that he was ready to depart the AMEX Stadium after three years on the south coast.
"The truth is they have been three years in Brighton with a lot of learning. But this will be a transfer market with a lot of movement," Estupinan explained.
"The club know that I would like to grow, that I would like to keep growing. The club have understood this.
"I've spoken to the club. The club have given me this bit of freedom to be able to make a decision. Now we will try to see better what is on the table and to see where we can go."
Estupinan would fit the Premier League-proven criteria that United appear to be targeting in an attempt to ensure signings can make an instant impact.
The Red Devils made Wolves forward Matheus Cunha their first summer recruit after triggering his £62.5m release clause.
Mbeumo also has a proven track record of scoring goals in the Premier League. United accelerated their interest after missing out on Liam Delap, who impressed in the top flight last season.
Amorim is well-stocked at left wing-back, with Patrick Dorgu, Harry Amass, Diego Leon and Luke Shaw at his disposal.
However, with Leon and Amass' senior experience limited and Shaw likely set for more minutes as a left-centre-back next term, another established option makes sense.