We've never played with a DM and wont whilst SAF is still in charge
Yes, you have, frequently, Keane was certainly the defensive minded player alongside someone like Scholes, DM's don't HAVE to be inept going forwards. Tiote is without question a DM, just because he can pass and run forwards(though he rarely gets that far and too date has one goal).
More recently (by recent I guess that is basically 3 years now) Carrick has been playing deeper, he's crap at it and one of the main reasons he's been so crap for a long time.
A DM doesn't have to go and put DM down on his birth certificate for it to count. Whoever the more defensive player in the midfield pair(or trio, or two players of a trio, or painfully two defensive players in a pair) IS the DM.
Just like, when Rio goes in goal because of a red card..... he's a goalkeeper, even though he isn't normally a goalkeeper.
Utd have always, and will always have a more defensive minded midfielder. Keane was brilliant at it, not least by encouraging fear of leg breaking by going past him.
Gilberto was a world class DM, and yet he scored 12 goals one season. Makelele was a DM and his game was to simply not go forwards, that doesn't make only players who play identically to Makelele a DM, that was just his version of it.
Utd have lacked a killer defensive minded, or defensive covering central midfielder for years. Fletcher keeps getting called the "spoiler" during his supposedly best time a couple seasons ago, what do you think he was doing, he was playing DM, this didn't stop him getting forward either.
Utd simply need new midfielders, and its natural in ANY football team to have one who is naturally gifted and very good at helping out defensively, and one who is excellent going forward. Lampard played as the DM for England for many years, and wasn't bad at it, nor brilliant but his offensive game suffered badly for England.
Had Utd had someone defensively minded in midfield, maybe they wouldn't allow so many shots on target and leave the defence so open, which has been their main problem this season, allowing more shots on goal than really ever before. I'll point out again that De Gea has averaged more saves per game than any of the main keepers this season and kept roughly as many clean sheets per game as well.