With Fletcher as well, Javi Martinez could play at CB when (not really an if as if he's been injured this long he's bound to have a few niggles) Vidic gets injured again.
Fletcher and Vidic though, after so long out don't expect them to work wonders immediately.
Javi Martinez is perfect, I've said that for a long time, he is the best young player in his position I'd say with M'Vila possibly in the world, the only difference being that Martinez is more tested at the higher level with the Spanish league being much tougher than the French.
Forget about Kagawa, bring in Modric and Javi Martinez and that is a sensational midfield.
Though I really don't agree that Modric is worth 40 m+. Not even close. If someone like Fabregas gets 35 m, Modric should surely get 30-35 m max, even if he whores himself out. However, it's United and big transfer fees are common for them.