SUNDERLAND are suing their former head of medicine and club doctor, Ishtiaq Rehman, over the medical that resulted in them signing Ricardo Alvarez from Inter Milan.

Sunderland officials are understood to have lodged legal papers at the High Court suing Rehman for more than £13m, a figure that covers the amount of money the club have paid out in fees since Alvarez signed in 2014.

The extraordinary legal action is part of a long-running saga that began when a clause in Alvarez’s contract forced the Black Cats to turn his initial loan deal into a permanent transfer, even though the winger had suffered a serious knee injury that would prevent him making another appearance for the club.

Sunderland are alleging that Dr Rehman was guilty of negligence and breach of conduct when he oversaw Alvarez’s medical in August 2014.

According to journalists at The Sun, who claim to have seen Sunderland’s legal documentation, the club’s lawyers claim Dr Rehman “did not oversee the medical appropriately, or act with best practice.”

At the time of Alvarez’s medical, Dr Rehman was in London helping Sunderland’s first-team players prepare for a match against QPR.

It is alleged he delegated the examination to four specialists, including a consultant who had extensive experience of dealing with knee injuries.

Alvarez, who suffered from chronic patellar tendinitis in his left knee, had undergone a cruciate ligament operation in 2009, and had also suffered a problem with his right knee.

Sunderland were aware of the history, but while a number of issues were raised in the immediate aftermath of Alvarez’s medical, the Black Cats claim that Dr Rehman emailed then chief executive Margaret Byrne and sporting director Lee Congerton to allay any concerns over the viability of the transfer.

Sunderland claim that while they subsequently entered an ‘exemption clause’ into the contract relating to Alvarez’s left knee, they were dissuaded from taking the same precautions with regard to his right.

Alvarez subsequently suffered a serious injury to his right knee at the start of the 2014-15 season, and was forced to undergo surgery in October 2014.

The South American did not play again that season, but having agreed to a clause in the loan deal with Inter Milan committing them to a permanent transfer at the end of the campaign provided they remained in the Premier League, Sunderland were placed in a difficult position when they successfully avoided relegation.

A lengthy legal battle ensued, with the Black Cats claiming the agreement was invalid because of Alvarez’s injury issues, but a ruling at the Court of Arbitration for Sport resulted in the club having to pay Inter Milan £9m, plus more than £300,000 in interest.

A further court ruling following a claim brought by Alvarez himself resulted in Sunderland having to pay a further £4.198m.

Dr Rehman, who resigned from his position at Sunderland after David Moyes withdrew Patrick van Aanholt from a game at Tottenham because of a cardiac issue, is still to respond to Sunderland’s accusations.