Normally I would have moved on from signing Sandro, but it does seem a quite bizarre decision to sign for Atletico. For a young player to take a voluntary 6 month break at quite a critical time in his career with regard to development and making a name for himself seems possibly suicidal.
Depending on how well they are doing he might not make it of the bench for the rest of the season either.
There is also talk he may be loaned back to Malaga until January, but how committed will he be to them. I cant imagine Atletico encouraging him to dive in for 50/50 balls and risk being injured long term either.