Former Republic of Ireland international Shay Given has returned to Aston Villa following a hugely successful loan spell at the Riverside Stadium, and Karanka now has to choose the right man to replace him for Saturday's Sky Bet Championship trip to Sheffield Wednesday.
Jason Steele, Jayson Leutwiler, Dimi Konstantopoulos and Tomas Mejias Osorio are the men in contention, and the Spaniard admits all four have a chance.
Karanka told the Northern Echo: "I'm not worried. It's better to have four goalkeepers ready to play and I need to choose the best one for the game.
"It's tough because they are all training well. I prefer it this way, I prefer to have a difficult decision. I have four in this moment and any one can play.
"It is not usual, but it is the situation I have. I have to choose the best one for the team, and I don't want to make a mistake."
Middlesbrough last week