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At the age of five, the youngster was diagnosed with Perthes disease, an extremely rare childhood condition that affects the hips.
He had to have a number of surgeries and was wheelchair-bound for 18 months. His parents were told by doctors that he may never be able to walk again, never mind play football.
But Declan wasn't having it and after defiantly joining in playing football in his wheelchair and on crutches while as a kid, his dream was realised in the summer when he signed a professional contract with Sheffield Wednesday.
And yesterday, in a 2-0 FA Cup third round win away at Exeter City, the youngster was sent on for his first-team debut.
His dad Lee, a former player with Boston United and now assistant coach with Stocksbridge Park Steels was overcome with emotion as he proudly watched on the TV.
Just a special moment for all the Thompson family and one that seemed very unrealistic all those years ago.
Now though, Thompson doesn't have any problems with the condition but is continuing to raise awareness.
"There's a young lad that has got it from near Wigan and I have been speaking to him and his family, it's helping them out a lot knowing that being told 'no' is not the end of the world," he told The Athletic.
"He can still become what he wants to become. My mum has had countless messages. She is in a Perthes Association group and all the parents in that said it had made their day.
"I was never really thinking about being an inspiration, I just wanted to be me and become what I've always wanted to be."
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