Derby County defender Mark O'Brien ruled out for the season

O'Brien signed a new deal in September running until 2015

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, who recently signed a new contract at Derby until 2015, 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.