Robben and Sneijder were Real Madrid cast offs as well and helped lead their teams to a treble within five years. People need to accept that, for now and the near future, Barca and Madrid will continue being the biggest attraction to players all over the world. Lots of talent pass through the Bernabeau and Nou Camp, most of them staying only 2-5 years in RM's case, and it's important to keep an eye on these players when they're sent packing. I know that most of you are traumatized over the Di Maria saga, but transfers fail half the time. It's just the nature of the game. No need to say no to someone like Kroos (who will instantly be a Top 4 CM in the PL) just because Madrid wanted to get rid. They are a circus after all.
EDIT: No offense meant, didn't meant to have go at you, just speaking in general with the Real phobia here
Going back to Mkhitaryan, he'd definitely be a great asset but at his age, he's not exactly a long term fix (tbf, same can be said for most of the players available in the market). I think he'd struggle once his pace and stamina declines, as he's not exactly blessed with elite dribbling or passing. Could be a Van Persie-esque transfer for United, get two years, maybe three tops, of great production from him and sell.