Rams defender goes under the knife to cure knee complaint.

Derby County defender Mark O’Brien has undergone knee surgery and is expected to be out of action for up to six months.

20-year-old O’Brien was in London on Tuesday, accompanied by Head Physiotherapist Neil Sullivan, and successfully went under the knife to cure a problem which has plagued him throughout the campaign.

The Republic of Ireland Under 21 international centre-back has been suffering from swelling in his knee throughout this season, which has been caused by a defect on the joint surface, and best described as a mini pothole on the joint cartilage.

This has been reacting to both training and playing matches. Rest and modified training have not helped ease the problem over the last few weeks, and O’Brien has now undergone surgery to repair the defect.

Manager Nigel Clough said: “It’s big blow for Mark and surgery is the only way to correct the problem he’s had, so we’re looking at between 4-6 months for him being out of action.

“It’s not ideal, especially with Shaun Barker out as well, but Mark is a mentally strong person and he will fight his way back, like he has done in the past, and be stronger for it.

“When he has been on the pitch for us, he has shown he is an asset for us and we continue to believe he has a bright future ahead of him.”

The news comes as another injury set-back for Academy graduate O’Brien, who missed the entire second half of last season after suffering cruciate ligament damage in training in December 2011, while he also underwent heart surgery in November 2009 after the discovery of a faulty valve and he also required ankle surgery during the 2010-11 campaign.

However, when the Dublin-born defender has been available for selection his form has been impressive and his form in the early weeks of 2012 earned a call-up to the Republic of Ireland Under 21 squad.

O'Brien has made 35 appearances for Derby in all competitions, with 11 of them coming this season.