FIFA has slapped a transfer ban on both Madrid clubs in response to their actions in signing and registering under-18 players. This is similar to Barcelona's ban. Specifically:
(1) it doesn't apply to this already-open window, but to the summer 2016 and January 2017 windows
(2) the clubs can still buy a player--but he can't be registered until the ban is over
(3) the clubs can still register players they own who return from loan, they can still promote players from their B teams, etc.
(4) Both teams can still SELL players; they just can't buy players and immediately suit them up.
So, what might this mean, both in this transfer market and the next two?
Obviously if the teams feel they are going to have needs over the next 18 months, they'd be best to buy a player now for that need and then loan him out for the remainder of the season--there's then no ban on bringing him back to the club in the summer.
As far as Denis Cheryshev is concerned, I'd say it might make a loan deal in this window more likely, since that way he could still be brought back to Madrid in the summer under the ban. If he's sold, unless they buy someone now, they can't do so (or, at least, can't do so and play him) until the 2017-18 season.
A player like Leo Baptistao can of course be returned to Atleti in the summer, or purchased; the ban doesn't affect that. I am not sure how the ban relates to players currently on loan to one of those clubs if there is a purchase option; my assumption would be he couldn't be registered in fall 2016. That might affect Fernando Torres; not sure about anyone else. But a player like Borja Baston, currently on loan to Eibar? Almost certain to move back and stay at the parent club, I'd think.
At any rate, it will be interesting to see the ripple effects in La Liga. Barca has shown these sorts of things don't have to affect results very much....