Marin was all set to join Villarreal on Monday's deadline day, but Manna admits Napoli's failure to sign a replacement in time forced them to drop the deal.
He explained: "He is the first player bought, replacing (Leo) Ostigard, for a growth path. We have invested, he comes from an important youth sector (Real Madrid), objectively we have defenders who are demonstrating a high level, he has to settle in, understand, even if 6 months have passed, we had the documents done with Villarreal who play to reach the Champions League and the coach wanted him.
"I had even promised him, he needs his path, the conditions were not there to take another and we continue like this, he has to work every day as if he had to play on Sundays. That's it, we have already spoken about it with him and with those who wanted more space. We are this and we play with XI but in reality we are all of us."