Man United had at least 17 players present at their training ground on Monday, with David de Gea, Donny van de Beek, Anthony Martial, Marcus Rashford and Jadon Sancho among those to have attended.
However, Garnacho was perhaps the main talking point considering that the 17-year-old was not expected back until July due to his involvement for Argentina Under-20s in the Toulon Tournament this month.
The Argentine last played on June 10 and was expected to have a three-week break following his international commitments, but the youngster has returned to training early in a bid to impress new head coach Erik ten Hag.
Garnacho made two Premier League appearances in the latter stages of last season, debuting against Chelsea at the end of April before featuring in Man United's final game of the campaign at Crystal Palace.
The South American starred at Under-18 and Under-23 level last season, meanwhile, and scored two late goals as the Red Devils won the FA Youth Cup by beating Nottingham Forest 3-1.
The attacker's contract at Old Trafford is due to run until June 2025, but a recent report claimed that he is in line for a new and improved deal in Manchester due to his development last term.
Garnacho, who has already been in a senior squad for Argentina, joined Man United's Under-18s from Atletico Madrid in October 2020.
He is capable of playing as both a left-sided and right-sided attacker and was named the Jimmy Murphy Young Player of the Year for 2021-22 due to his impressive form at youth level.
Garnacho is expected to be on the plane for Man United's pre-season tour of Thailand and Australia and will be looking to make a strong impression on Ten Hag with a view to receiving more first-team football next term.
According to the Manchester Evening News, fellow youngsters Ethan Laird, Alvaro Fernandez and Shola Shoretire also trained with the main group on Monday.
A number of familiar faces were missing, though, with the likes of Harry Maguire, Raphael Varane, Scott McTominay, Fred, Bruno Fernandes and Cristiano Ronaldo given extra time off due to their international commitments this month.
Dean Henderson was not pictured at the Carrington base amid suggestions that he is closing in on a switch to Nottingham Forest, while it is unclear whether Eric Bailly or Axel Tuanzebe were involved in the session.
Man United are expected to fly to Bangkok are July 8 ahead of their first pre-season fixture against Liverpool on July 12, while they will also take on Melbourne Victory, Crystal Palace, Aston Villa, Atletico Madrid and Rayo Vallecano as part of their preparations for the new campaign.