AITOR KARANKA claims Middlesbrough have completed “80-90 per cent” of their summer transfer business – but the club are in the process of stepping up their search for a new centre-half following the collapse of Neven Subotic’s proposed move from Borussia Dortmund.

Boro pulled the plug on an £8m deal for Subotic when the defender’s medical revealed a chest problem that will keep him out of action for at least the rest of the year, but Karanka remains determined to add another experienced central defender to his squad.

As things stand, Boro’s senior centre-half options consist of Ben Gibson, Alex Baptiste and injured duo Daniel Ayala and Bernardo Espinosa.

Espinosa has not played since damaging his knee ligaments in January, while Ayala is currently sidelined with an ankle problem and continues to be linked with a possible move away from the Riverside, so there is clearly a need to reinforce the heart of the back four.

Boro have closely monitored a number of defenders in the last 12 months, and are in the process of revisiting some of the targets that were shelved when Subotic emerged as Karanka’s preferred option at the start of the summer.

The addition of another centre-half would take Boro’s summer strengthening to eight players, and while a number of fringe performers are likely to leave before the transfer window closes, either permanently or on loan, Karanka does not envisage much more incoming business between now and the end of August.

There is perhaps scope for another attacking midfielder, or increased competition for first-choice left-back George Friend, but unlike in previous seasons, when Boro have been busy on deadline-day, Karanka is delighted to have completed the vast majority of his transfer business in the early stages of the summer break.

“I am very pleased because, in the last two seasons, when I have been here (in Marbella), we were training with 50 or 60 per cent of the team,” said Karanka, who will next lead his side into action when they take on Spanish side Alcorcon on Saturday evening.

“Now, I don’t know whether the number is 80 per cent or 90 per cent (of transfers completed), but nearly everything is done and that means the club is working well behind the scenes.”

The most recent addition saw Alvaro Negredo complete a season-long loan from Valencia, and the former Manchester City striker should be involved in the final two matches of Boro’s Spanish camp.

Negredo is desperate to reignite his career after a difficult two years with Valencia, and while Jordan Rhodes and David Nugent both played prominent roles in securing promotion last season, the 30-year-old Spaniard is expected to start the new campaign as Karanka’s first-choice forward.

“He is a really good player, and if he recovers his best he will be a really important player for us,” said the Boro head coach. “Another reason I wanted to sign him is because he has played in this country and he knows the league.

“I’m pleased (he signed) and he’s pleased because he wants to play his best, and he knows that England and this club is the best place to recover his self.”

Meanwhile, Subotic has outlined his despair at his latest injury setback, having spent the second half of last season recovering from a deep vein thrombosis that was diagnosed in his arm.

“I wanted a challenge I can actively work on,” said the Serbian centre-half. “I now have it, but unfortunately it’s not on the football field.

“In spite of the victory against the thrombosis and the time invested in preparing for the new season, it has become clear I have to undergo an operation.

“It’s hard on me because I was looking forward to a new season on the pitch like a small boy who can finally live his dream again. What remains is my dream, but a lot of hard work lies ahead of me.”