Garner has been mentioned as the next Michael Carrick and was given his first-team debut under Ole Gunnar Solskjaer earlier this year.
The youngster only just turned 18 this week but after making an appearance against Crystal Palace, the Norwegian displayed grand trust by using him vs Paris St-Germain in the Champions League.
And now the Englishman has penned a new deal, keeping him at the club until June 2022 – with an option to extend by a year.
But the Birkenhead-born prospect has drawn criticism from fans on social media for his deadpan expression when signing a new contract.
In the tweet announcing the deal, Garner stared down at the camera showing a blank face.
One fan responded: “Bloody hell lad cheer up your signing a contract to the biggest club in the world, di Maria was happier than the way u look when he signed.”
Another sarcastically said: “The excitement on his face says it all.”
While this fan said: “He doesn’t look happy about it.”
“Cheer up mate you’ve just signed a contract,” another said.
And this supporter tweeted: “He could at least look happy about it.”
However, speaking to the United website, Garner could not hide his delight as some fans had suggested – insisting that it was a special day for him.
“I’m made up,” he said.
“Obviously, it’s what I’ve been working for since the Under-8s and Under-9s. I’m proud for myself and for my family but I think it’s only the start. My mum, dad, grandad and auntie have all come along and it’s just a proud moment.
”I think it’s even more special to sign at Old Trafford. It would still have been special signing at the Aon Training Complex but this is even more special.
“Winning the league, having a good season and being nominated for the Young Player of the Year and now this just caps it off as a perfect season for me.
“I’m off for a break but will work hard in the close season. When I come back for pre-season, hopefully I’ll hit the ground running and be ready for next season.”