His inability to break up play his conservatism and general lack of inspiration.imagination isn't.
When compared to the best players in the world in that role past and present he doesn't match up at all and as long as he anchors UTD's midfield they'll never be solid/stable.
In fairness Michael Carrick has never been a destroyer, or a defensive midfielder. He is a deep lying player. In the mould of a Busquets, or Arteta, or Pirlo (without the goal scoring ability) . Keeps possession tidily, great touch and passing, and sprays balls out to the wings and flanks. Every once in a while he'll step in and break up play,but its not really what he does best. The reason United are struggling are as you said he is asked to do this role that he isnt suited to. He needs a Darren Fletcher, a box to box player, or a dedicated defensive mid beside him to be at his best. I do think Michael Carrick has been one of England's most "continental" type players since the Premiership started. Its a shame he hasnt gotten more games for the 3 Lions.