Maguire, 29, has found himself on the bench this season with Raphael Varane and new signing Lisandro Martinez largely preferred at centre-back by United manager Erik ten Hag.
He now faces a battle to keep his place in the England team for this winter's World Cup in Qatar.
Despite his current struggles at club level, Maguire was included in Gareth Southgate's latest England squad for their upcoming Nations League matches against Italy and Germany.
Speaking ahead of Friday's match between Italy and England at San Siro, former Azzurri captain Chiellini gave Maguire his backing and believes the centre-back is being unfairly criticised because of his £80m transfer fee, which United paid to Leicester in 2019.
Assessing England's defensive options for this weekend, Chiellini told The Times: "I am sad for Maguire's situation because he is a good player.
"They [Manchester United] require too much of him. Just because they paid £80m for him, he has to be the best in the world every match? It's not right.
"The value of the market is dependent on many aspects you can't control. It's not your fault.
"Him and [John] Stones are a good duo. OK, Maguire maybe is not Rio Ferdinand but he's good enough. With this situation it's not helping [England] to do the best.
"If you want to win the World Cup it's impossible to do it with some problems in the key players, and for sure Maguire is one of the key players of the team."
Fikayo Tomori has emerged as a potential candidate to replace Maguire in the England team after winning the Serie A title with AC Milan last season.
"He surprised me because I saw him in Chelsea and I didn't think he could be so strong in the box [or] attentive for 90 minutes," added Chiellini, who was part of the Italian side that beat England in the Euro 2020 final at Wembley.
"He is one of the keys of Milan winning last year."
But the veteran defender, who announced his retirement from international football earlier this year, reserved his highest praise for Manchester City defender Kyle Walker.
"I love Kyle Walker," Chiellini said.
"Walker is unbelievable. I don't know how it's possible to be so fast, so strong. He could do the 100 metres against jaguars, maybe, because he is something fantastic."
Who should England start at centre-back at the World Cup this winter? Let us know your thoughts.
SPORTbible Stories is the brand-new podcast from the SPORT team at LADbible. The first episode features a chat with former Burnley manager Sean Dyche as he opens up on his departure from Turf Moor, what the future holds and the amazing story of the spin wheel for fines while in charge of the Clarets. You can watch the first episode here.
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