You must value Lamela VERY highly in that case!
Selling those two for £50 million would be an incredible piece of business and allow us to invest that £50 million (and hopefully some more) in replacing them with players who are good (and fit) enough to help us win trophies. Lamela is an admittedly borderline good enough sicknote. Dier is just not good enough, slow on the turn, immobile, average to poor control, can't carry the ball, doesn't pass particularly well, commits a lot of fouls (I think probably because he is so slow now so just always wrong side and/or late).
In every PL game that I watch the opposition seem to have a deep central midfield player who is better than Dier. I think he would be incredibly easy to replace. We could sign Kalvin Phillips from Leeds for £20 to £25 million and instantly be far better in both defence and attack than when we have Dier lumbering around doing little other than conceding fouls.
Lamela I admit does have quality and he also matches that with work rate. If we don't consider fitness then he would be a starting midfield player for us. The problem is that he is fit so rarely. Can we afford to keep relying on a player who we expect to be available for less than 50% of the time?
So if we've aready improved on Dier using only half of that £50 million the question becomes whether we could buy somebody to replace Lamela for the remaining half?