Good shout about the medical department, will be interesting to see what other changes Frank makes this summer. I could also easily make the argument that the value Mina would provide in the 15 games he plays next season might be more valuable than the £5-10m we would get for him. It's his high wages that complicate it. Plus it's incredibly disruptive to the defensive organization to always have your best CB injured. Now if we could get to a position where he's our 3rd best CB then it's much less of a problem, but I seriously doubt that can happen this window. The best course is probably to free up the wages for someone who is more durable like Tarkowski (even if it is a step down in ability).
The disruption at the back under normal circumstances is manageable, the major problem us when the disruption is due to the guy who actually is the only organizer/leader at the back being the one whose constantly missing - so leaves a group of players needing organization with no one able or willing to do so.
It's why Tarkowski is a good signing. Keep Mina you have two vocal players who can sort out the backline. So should guarantee at least one will be there every week to babysit the likes of those who remain.