The 18-year-old, who was initially brought on to replace Jadon Sancho with 17 minutes remaining in the game, was forced off in injury-time, not long after bearing the brunt of a heavy, but fair, challenge from Southampton full-back Kyle Walker-Peters.
After initially playing on, Garnacho was replaced by midfielder Fred in the closing stages and then hobbled along the touchline and back through the tunnel. He was later seen leaving the ground on crutches after the game.
Erik was keen to ease fans of their initial worry regarding the forward's situation, when asked for an update in his post-match press conference.