I get the feeling that in Nico's case he really wanted/wants to succeed here. He seems to be too much in his head about proving himself, which makes his decision making poor. Technically he is one of our better players. His injury record will always be an issue though.
I would only sell if we could recoup (close to) what we need to avoid a loss, otherwise just keep. Especially if the only plan would be to replace him with Sancho last minute.
Some things need to fall in place in our favour for this season to be somewhat of a success. Bremer being fit is the most important one, but players like Nico (if he stays) and Koop having the impact we expected in their first season would be a boost as well. Wishful thinking, but that is the situation we are in.