The young Japan winger has impressed with his performances in the League Cup but is set to be for a spell on the sidelines after limping off the field in a reserve game
It is a disappointing turn of events for Miyaichi after Arsenal manager Arsene Wenger stated that the teenager would be presented with more first-team opportunities.
Reserve team coach Neil Banfield told the club’s official website: "Ryo has sprained his left ankle and will be out for a minimum of three weeks."
Miyaichi, 18, is seen to be one of the Gunners' brightest talents and has shown glimpses of his ability this season in League Cup appearances against Shrewsbury and Bolton.
The teenager also enjoyed a promising half-season loan stint at Dutch club Feyenoord last term, making 12 appearances, scoring three goals and providing five assists.