Teenage sensation Garnacho was an unused substitute in United's 3-2 win over Norwich City on Saturday afternoon, a game dominated by Ronaldo.
The 37-year-old scored a hat-trick and gave his match ball to Garnacho at full-time before embracing the youngster.
Garnacho posted an image of the pair captioned: "Greatest of all times..." along with goat emojis.
It saw former Manchester City and Argentina striker Aguero reply with: "Because you still didn't play with the best pa @leomessi."
The forward was called up to Argentina's senior squad for last month's internationals against Venezuela and Ecuador but didn't play against either.
Garnacho had that comment pinned on the post initially, until Ronaldo himself showed appreciation with a fist emoji.
He immediately pinned that ahead of Aguero's which fans on social media noticed.
Garnacho removed Aguero's pinned and look whose comment he pinned.— Geo (@GeorginaXeven) April 17, 2022
Cristiano Ronaldo 🐐. pic.twitter.com/3zjGVlZBSE
An Argentine player preferring Ronaldo to Messi. We live in strange times ladies and gentlemen.
Garnacho could find himself in the exclusive club of players to have played alongside both when his career is said and done.
Regarded as one of the top prospects in United's academy, he's been tipped to make a breakthrough sooner rather than later.
He's featured for both the U23s and U18s this season and been a vital part of the latter's FA Youth Cup run.
Argentina called Garnacho up to their senior squad to ensure Spain didn't get their hands on him.
United interim manager Ralf Rangnick discussed his inclusion in the weekend's squad and spoke fondly of his talents.
He told the club's official website: "[Garnacho] has been training with us [the first team] for the last two weeks and I know that he is one of the most promising players.
"[He is] still only 17, but a player that hopefully if he keeps his feet on the ground. If he keeps trying to get better every day, then he will not only be sitting on the bench but there is every chance he will be playing for this club."
United are fifth in the Premier League and find themselves back in the top four race after Tottenham Hotspur and Arsenal's defeats to Brighton and Southampton respectively.
They face bitter rivals Liverpool at Anfield on Tuesday evening.
Featured Image Credit: Sergio Aguero/Twitch & Manchester United
- Sergio Aguero Trolls Alejandro Garnacho After Classy Cristiano Ronaldo Gesture
- "It is decided..." - Avram Glazer breaks silence on Manchester United sale and Cristiano Ronaldo's departure
- "Huge headlines" - Ronaldo's bombshell interview with Morgan contains thrilling Messi debate
- Saudi Arabia fans mocked Argentina supporters with Cristiano Ronaldo's 'SIUU' celebration outside stadium