Oubina, on a season-long loan from Celta Vigo, suffered the injury after 13 minutes of his City debut against Liverpool on Saturday and an MRI scan has revealed damage to his anterior cruciate ligament, tibial collateral ligament and medial ligament.
Birmingham boss Steve Bruce said: "It is a bitter blow for us - but more importantly for the lad himself."
Copyright (c) PA Sport 2007, All Rights Reserved.
Do YOU want to write for GiveMeSport? Get started today by signing-up and submitting an article HERE: https://www.givemesport.com/writeforgms