I honestly think he is more dynamic than Alonso. He likes to get forward a lot more, rather than sitting deep and playing the 'quarter back' role that Alonso plays. To roam forward though, he needs the holding midfielder alongside him. I'm not sure Alonso and Sahin would work as a midfield partnership, but I really hope Mourinho doesn't put the shackles on his forward running in trying to replace Alonso for the future. I'm really intrigued to see how Mourinho works his midfield next season.
I do think he'd have been perfect for United though. He has the physical ability to cope with the demanding speed of the premiership and the technical ability to shine. He'd have been a real fan favourite