Man United starlet Garnacho will be out for several weeks.
The 18-year-old will miss two of Man United’s games this week and be denied an Argentina debut in the international break.
In the Southampton game, Garnacho came on in the 73rd minute but was taken off in extra time after he was hurt in a challenge with Kyle Walker-Peters.
He left the Old Trafford Stadium on crutches and the club is waiting for an exact diagnosis of the ankle injury.
After scoring four goals and making four assists this season, the youngster was expected to make his debut for the Argentina team.
Argentina faces Panama and Curacao in friendly matches later this month during the international break.
Garnacho wrote on Instagram: “It is difficult to put into words how I’m feeling right now.
“Unfortunately I will not be able to help my team and teammates in the upcoming games in what is a very important part of the season for us at Manchester United.
“I am disappointed to also miss the opportunity to be with my teammates of the Argentinian national team in what would have been a great and proud moment for me and my family.”