The teenager scored with a stunning strike to give United a 1-0 win over Real Sociedad in their final Europa League group game.
The strike was made even more special for the 18-year-old given how it was his 'idol' Ronaldo that put him through on goal, capping off a 'dream' first goal for his club.
Alejandro Garnacho (left) asks Cristiano Ronaldo's (right) permission to do his celebration
A laughing Ronaldo gives his blessing, completing a 'dream' moment for the 18-year-old
The young forward then performs Ronaldo's new 'napping' celebration in front of the fans
As he celebrated with his United team-mates, Garnacho can be seen asking Ronaldo if he could use his new celebration while gesturing it to the Portuguese talisman.
The heartwarming moment makes Ronaldo laugh, before he pats Garnacho on the head and grants him his wish.
A moment later, the young Argentinian pulls away from his United team-mates and is seen paying tribute to the five-time Ballon d'Or winner.
Garnacho took to social media after the game to thank Ronaldo for his part in making it a night he'd never forget.
Ronaldo has ditched his trademark 'SIUUU' celebration in favour of his napping one
Garnacho tweeted: '18 years and 125 days dreaming of this moment. Thanks Idol, @Cristiano'
Garnacho's strike also earned the plaudits of United greats Paul Scholes and Owen Hargreaves, who were full of praise for the young forward.
Speaking on BT Sport at half-time, both Hargreaves and Scholes credited him with being United's best in another encouraging performance.
'I'm really pleased for the young lad, he's been United's best player,' Scholes said.
'That second touch really makes the goal, he gets it out of his feet and slides it into the top corner. I think he's been far and away the biggest attacking threat on the pitch. I don't think the keeper covers himself in glory, but it's a lovely finish.'
Garnacho thanked his 'idol' Ronaldo for his part in making it a night he'd never forget
Meanwhile, Hargreaves added: 'That's the great thing about young kids, they play with a real enthusiasm. Every time he touches it he runs forward, the first touch is amazing. It is great to see him mentored by Cristiano [Ronaldo].
'He's a really good player and it's nice to see him performing on the biggest stage.'
Ronaldo has recently ditched his trademark 'SIUUU' celebration in favour of his new sleeping one, which is said to be a self-deprecating in-joke between Ronaldo and his team-mates, according to Manchester United's official website.
Ronaldo is renowned for keeping himself in good shape off the pitch and also ensures that he gets plenty of rest, with the celebration representing the position in Ronaldo sleeps when he is travelling with the squad.
The 37-year-old used his new celebration after scoring his 700th goal of his career in a win over Everton last month, while his impact on the footballing world has seen the celebration being used by other players – and even young mascots.
The Argentine scored his first goal for the club to put United 1-0 up against Real Sociedad
The teenager gets his moment with the away support as Ronaldo looks back at the youngster
While it will have been another dream come true for Garnacho, the same can't be said for United and Erik ten Hag, who fell short in topping their Europa League group.
United needed to win by a two-goal margin against Real Sociedad to finish top and guarantee their spot in the knockout stages.
United will find out their play-off opponents on Monday.
اخلاء مسئولية! : هذا المحتوى لم يتم انشائة او استضافته بواسطة موقع اخبار الكورة و اي مسؤلية قانونية تقع على عاتق الموقع مصدر الخبر : dailymail.co.uk  , يتم جمع الاخبار عن طريق خدمة ال RSS المتاحة مجانا للجمهور من المصدر : dailymail.co.uk  مع الحفظ على حقوق الملكية الخاصة بمصدر الخبر.