There's no question we are weaker without him. He and Jan are equally good. Both of them outstanding defenders. We have good options but none as good as him to replace, and any new arrival will be uncertain. Turning Utds second best defense into the best doesn't seem wise to me if we want to beat them to the title.
I'd keep him a year. If worst happens and someone gets him for 25M next summer when he is a year older and we've had another season out of him, that's not the end of the world. Not like he'd leave on a free. If he goes on a free two years from now age 31 then again that's not the end of the world.
Plus every club in Europe would want him at that price a year from now so if he is injury free he'd sell, and more likely to be sold abroad.
If he gets injured or has dodgy form then he may even stay. Maybe he chooses to stay after we win the league. No one knows how things could be a year from now.
Selling to a rival years ago made sense, we weren't close rivals. But now it may well decide who gets the title ahead of us.