Alonso does the main defensive job for Real Madrid. Kedeira does the running in front of him and supports the attack. Sure he's combative and puts tackles in but Alonso is sitting and its Kedeira in front of him
Sahin could absolutely be deployed next to him as Sahin, like Alonso, has both defensive and offensive qualities. He's not as good defensively as Alonso is for Madrid, but he proved to me that he can put in some very good tackles when Dortmund played Bayern. He had a spell where he was making good tackles every couple of minutes and stopping Bayern getting back into the game.
And offensively he will assist as much as Alonso, but also score more goals. This is a player who at a top team will flourish into one of the top midfielders in the world. Better than Alonso, catching up towards the Fabregas and Iniestas. But they wouldnt have to compete for one place.
As for Mourinho, we've already seen him use Kaka as one of a midfield 3. He isnt always negative. Not with the players he has at Madrid
Compared to that this wouldnt even be a risk