Birmingham's worst fears have been confirmed with on-loan midfielder Oubina ruled out for at least six months with knee ligament damage.
Oubina, on a season-long loan from Celta Vigo, suffered the injury after 13 minutes of his City debut against Liverpool at Anfield on Saturday.
The 25-year-old had an MRI scan which has revealed damage to his anterior cruciate ligament, tibial collateral ligament and medial ligament. It means Oubina will be out of action until at least March and he is currently back in Spain having treatment for the injury.
Birmingham boss Steve Bruce said: "It is a bitter blow for us - but more importantly for the lad himself. From what we had seen of him in training, it was obvious that he is a very good player and to have his season cut short like this just minutes into his debut is tragic."
Bruce has been fated by injuries to key midfielders since he took charge at St Andrew's including David Dunn and Muzzy Izzet. He had been hoping Oubina would have provided creative qualities in the centre of the park after previously missing out on signing Mathieu Flamini and Hossam Ghaly from Arsenal and Tottenham respectively.
Oubina said: "This is terrible for me because I arrived in the Premier League with much anticipation. I wanted to demonstrate that I would be able to play at that level. From the first moment I got the injury, I feared the worst."
Oubina suffered the setback in an innocuous-looking challenge from Liverpool's Dirk Kuyt.