NIGEL Clough has praised the "amazing" resilience of Derby County's young defender Mark O'Brien.

O'Brien is currently sidelined with a serious knee injury but the 19-year-old is already slightly ahead of schedule on his comeback trail.

He suffered anterior cruciate ligament damage in training in December. He underwent reconstructive surgery and was ruled out for the rest of the season.

It is not the first time the Republic of Ireland youth international has suffered a serious injury blow in his short career.

He had heart surgery in 2009 to repair a valve problem and last year had an operation on his ankle.

O'Brien recovered from both set backs to win a place in the first team.

And Derby are confident he will bounce back better and stronger from his latest blow.

"Mark is coming on great. He's off his crutches and well into his rehab now," said Clough. "He is a bit ahead of schedule, which doesn't surprise us given his character.

"He was off his crutches first chance he got and in the gym.

"His resilience is amazing for a 19-year-old. That is partly down to what he has gone though in the last couple of years but mainly down to his character and his upbringing.

"Whatever is thrown at him he just gets on with it.

"He bats off setbacks. He has the mental strength to say 'I am going to come back better and stronger from this'.

"And he will do.

"Some injured players you have to push and cajole but Mark is one you have to hold back.

"He will be wanting to run as soon as he possibly can, so we just have to be careful."

Derby said the specialist is delighted with O'Brien's progress and the player should be running again in April.

"The way it has worked out has not been too bad because Mark won't be ready until the end of the season and so he will then have a couple of months to get ready for pre-season," said Clough.

"He should be doing some good running in April and May and the main thing is he will do a good, full pre-season."

O'Brien was 16 when he made his first-team debut in Derby's final game of the 2008-9 season at Watford.

He made 16 starts earlier this season and featured in 21 of 24 league and cup fixtures before his knee injury.

He signed a new four-year contract in September.