Middlesbrough defender Daniel Ayala could be out of action for up to six weeks.
Head coach Aitor Karanka has revealed the ankle injury the 23-year-old, who completed a permanent move to the Riverside Stadium from Norwich last month, suffered against Doncaster last weekend is more serious than was first thought.
He told the Evening Gazette: "Dani has a serious injury, he has a problem with his ankle.
"I think it will be four, five or six weeks. It is the injury he had at Doncaster. I thought - and he thought - it wasn't a serious injury, but finally we now know it is a serious injury."
The news came as a major blow to fellow Spaniard Karanka after he won a protracted battle to clinch Ayala's signature after a successful loan spell.
He said: "We were fighting to get his permanent contract. We managed to do that, which was good, but now he is injured - that's football."