A scan on Abella's leg this week confirmed the Spaniard's initial fears, with the Boro coach telling reporters: "It is his ACL. We thought at the game it was a serious injury. Normally this kind of injury is six months. I suffered it when I was a player and it was six months.
"He is an experienced player and he is obviously not happy, but he was better than all of us and was the first to understand his injury. He's a very good lad. He will have an operation and then rehab and we'll hopefully have him back as soon as possible.
"He is the kind of players I want in my squad always. He is a good lad and the kind of professional that works hard whether he is playing or not."
Middlesbrough are currently ninth in the Championship table.