The Croatian Tin Jedvaj is a good Central Defender. I managed to get him for 2.4-million because Roma transfer-listed him after a poor loan spell with Ajax, but he's worth more if you have to pay it. And what about Dejan Lovren? If your database is updated and he's already at Liverpool, then maybe not, but if he's still at Southampton you might be able to get him. Mark O'Hara and John Souttar (both Scottish) are both decent centre-back prospects but may already have signed for other top clubs in your save. I'd also look to the Dutch league, Barcelona reserves and the likes of Genk and Anderlecht in Belgium for young players who have the potential to become Premier League greats.
It sounds as though Huth is still under contract to you, so it might be an idea to sign a young replacement who you can loan back to his original club for a season so he can continue his development - the process of getting his Current Ability closer to his Potential - while Huth plays out his last year. That way, the player that eventually joins you will be that much more the finished article. Only works if the young player in question is already an established first team player at his original club, though.
I did this with a young Belgian defender - a regen called Roel Rutten - who I signed from Genk. I loaned him back to them while the Mexican Hector Moreno played out his last season, allowing Rutten to play a further 50 matches in all competitions during his final season in Belgium. When he joined me the following season he was virtually fully developed and he helped me to win the Premier League for the first time in his first season. He has gone on to win the World Cup, European Championships and Confederations Cup with Belgium, and the European Champions League, Europa League and European Super Cup, plus a shed-load of domestic honours, with my Southampton team. I'm currently trying to repeat the trick with a young Spanish defender who I have loaned back to Real Sociedad, where he is playing every match.