Los Colchoneros will look to take advantage of Mata and De Bruyne's World Cup predicaments. With both players now struggling to hold down first-team places, their spots at next summer's World Cup could be in jeopardy.
Loan deals would be most likely in January, but permanent deals would not be out of the question.
The Blues would be able to absorb their losses due to the awesome depth they possess in the attacking midfield areas, but whether Jose Mourinho would be keen to lose some of that at the halfway point is unclear.
Mourinho seldom allows players to leave in January unless they are completely disposable or replacements are locked up.
Replacing players of that quality in January will be tricky, especially those who can play in the Champions League still.
This leads me to believe any deal for the moment is unlikely.