Dembele playing up as he seeks to move to Barcelona.
"We will sort this out behinds closed doors," said Castro.
"But we must say clearly that this cannot happen and that his behaviour is bad for the team.
"If a player can say 'as another team wants me I won't train any more', that is terrible for football."
It was a view also expressed by fellow defender Sokratis.
"Ousmane is a young kid but he needs to realise that no one player is bigger than the team. We all need to understand that we need to work for the team," he said.