No that's just a daft assesment and clearly not accurate. It's because he reads the game better, is superior at dictating play, positionally he is exceptional and he can dribble a ball better than Sahin. His close control is just superb, look at the two closely and you will see what I'm getting at.
I think Mourinho will hit gold with Modric, he's going to give Real Madrid even more of an edge. If Sahin was better, why would Jose want Modric? Doesn't make sense and I think Mourinho knows an exceptional midfield player when he sees one.