the worst part of sahin leaving madrid for good was that he actually played to expectations when he played midfield. Why didn't you sacrifice alonso or khedira for sahin mou? Biggest mistake mourinho ever made for madrid was his handling of sahin.
If Klopp manages to restore sahin to his true self.... Dortmund would have a midfield that not even bayern could match. he's that good.
That's simple.
Sahin can't play without a bit of muscle alongside him. He's defensively poop, and softer than Januzaj, so you couldn't just stick him and Alonso in the same side. Alonso is the better playmaker, too. It makes no sense to drop Alonso for Sahin, it might now, or in another year, but not back when Sahin joined.
The real issue, and something that will once again happen at Madrid, is buying players that aren't needed, or who don't fit into an exact plan. Look at Modric, he was a superlative player at Spurs, he's lacking confidence and consistency at Real. I worry for his future, especially with Illaramendi coming in.
And it's partly Sahin's fault, too. The team was built around him at Dortmund, he didn't realise what he had until it was gone.