Selling digne isn't an issue if the lad replacing him is good. Being younger then you would hope he has a better ceiling.
The problem I would have is if we sell for a similar price or loan out to a smaller fee in the summer.
There is nothing wrong in selling our best players and replacing them as long as we make money from them in the process. We aren't in a position to keep international footballers whilst the form isnt there. But the minute we are directly replacing players who are our best and making no profit from it, that's when it's not a good idea.
New lad may be boss in the future but you need that quality now, not in 3 years when it is too late.
It would be similar to selling lewen for 30 million and signing 18 year old lewen for the same price , he may go on to score loads for us but not straight away, so we lose the goals.
Selling digne loses the experience at fullback. Plus I can't see past the notion that it's benitez management that may be detriment to his form rather than the lad suddenly being average.