Mainoo has enjoyed a breakthrough campaign under Erik ten Hag, emerging as a rare shining light in a difficult period at Old Trafford.
The 18-year-old started his tenth consecutive match across all competitions to help United beat Aston Villa 2-1 on Sunday.
Mainoo opened his Premier League account after scoring a dramatic stoppage-time winner in a seven-goal thriller at Wolves earlier this month.
The midfielder has represented England at under-17s, U18s and U19s level, but Ghana, who he qualifies to play for through his parents, are still keeping tabs on him.
According to the Daily Mail, the FA are aware of the Black Stars' wish to convince Mainoo to switch international allegiances.
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The report adds that the FA want Mainoo involved in England U21's European Qualifiers against Azerbaijan and Luxembourg.
Young Lions head coach Lee Carsley is said to have been keeping close attention to the teenager's progress in recent weeks.
Gareth Southgate oversaw Mainoo's performance at Villa Park last time out and there's confidence he will opt for England over Ghana.
The United academy graduate only started his first Premier League match in November, as he dazzled in the 3-0 away win at Everton.
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United boss Ten Hag recently lifted the lid on Mainoo's development, revealing that he expects him to benefit from playing with Casemiro.
He said: "You can see it. We missed, for over a long period, very important players like Casemiro, like Lisandro Martinez, like Luke Shaw, also others.
Rio Ferdinand compares Manchester United's Kobbie Mainoo to Clarence Seedorf after injury-time winner against Wolves
"We discuss that often, but you see the impact of a player when he's there. Like Casemiro, he gives you calmness and composure on the ball, he gives you options, he sees the vertical pass.
"He is always a step ahead of his opponent and breaks lines.
"We talk just about the offensive, how they have to serve, but then Casemiro is a very important player for our game so we're happy he's back.
"For Kobbie Mainoo, he can learn a lot from it and that togetherness, that bond… also Kobbie is very coachable and Casemiro has so much experience.
"Kobbie will learn a lot from it and that can even accelerate his development.
"So yeah, I'm happy to have him back and when they can play together I think they can take benefits from each other."