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Derby County's teenage defender Mark O'Brien has been ruled out for the rest of the season after damaging cruciate ligaments in training on Tuesday.

The 19-year-old Irishman, had surgery on Wednesday after visiting a knee specialist.

The centre-back has made 21 appearances for Derby County this season.

"It's a big blow for Mark and we're all devastated for him," said Rams manager Nigel Clough.

The Dublin-born defender has already suffered a number of setbacks in his career.

He had heart surgery in 2009 at the age of 16 after the discovery of a faulty valve and he also required ankle surgery last season.