The electric attacker came on in the second half of our 0-0 draw with Southampton on Sunday and underwent treatment after being hurt in a challenge with Saints right-back Kyle Walker-Peters.
The 18-year-old played on following the tackle, but was replaced by Fred in injury time and headed straight down the tunnel.
Speculation on the winger's fitness ensued and on Tuesday afternoon, Garnacho revealed that he faces a spell on the sidelines and misses out on the opportunity to represent Argentina's national side in the international break.
He shared his disappointment on Instagram: "It is difficult to put into words how I’m feeling right now.