I have watched Manolas a number of times as he plays in Olympiacos, which is the team I support in Greece. He was always considered a great talent, but since he came in Olympiacos (which is the only Greek team that plays normally in CL) he has greatly improved. He is still inconsistent, as he is rather young for a defender, that's why I don't feel very safe when he is paired with another not so experienced defender. When however he is paired with someone who can has some experience, he can do great things. Although he is still inconsistent, the improvement he has made in the last year, is unbelievable. From the matches against Manchester to the World Cup, he has been great. He is NOT a commanding defender (not for the moment at least, maybe in the future) but he has great physical characteristics and he also has a lot of what we call football intelligence.
He is a gamble. He continuously improves. Especially the last 10 months - a year have been incredible. He is still only 23 years old. If he can keep it like this, he will be world class one day. But can he? If we believe yes, then around 8 million would be a fair price.