The 18-year-old has often been electric this season but his appearance off the bench against Southampton on Sunday was cut short early, after he suffered an injury during a challenge from Saints' right-back Kyle Walker-Peters.
Garnacho continued playing for a little while longer, but he was eventually substituted for Fred during injury time in our 0-0 draw.
Our no.49, who has been called up by Argentina for the upcoming home friendlies with Panama and Curacao, was later seen leaving Old Trafford on crutches.
Anthony Martial and Marcel Sabitzer are also injury doubts for the trip to Seville, while Erik ten Hag will hope to have Victor Lindelof available again after illness.