We don't use defensive midfielders, we never have really under Fergie. People will say that Roy Keane was a defensive midfielder but the fact is, if you even remember, Keane was an all rounder like Robson when we first bought him, and was scoring goals. He turned into a more defensive player as he developed and when he was partnered with Scholes.
Teams that play the modern way, usually that play a 433 formation, rely on a defensive midfielder to allow the other 2 midfielders to attack alot more. Usually this defensive midfielder is a ball winner and doesnt get forward alot. Martinez and Busquets are good examples and to be honest, theyre more like centre backs that play in midfield.
Alot of people want us to develop a more modern style (myself included) and have a midfielder like this but its not gonna happen. If we buy a midfielder at all, he will more than likely be like the midfielders we have - not defensive and not too attacking but somewhere in the middle. Let City have Martinez. We will go for Witzel