I think he had a few poor positioning moments even as CB, but as FB he often forgot his man totally. He remonds me a little of Bailly in that sense, unaware of what is going on behind him and not always making the right decisions. But again, this would most likely improve with matches. He and Lindelof have different styles. Axel has qualities that you mention in your other post like stronger and better tackler ( think he looks surprisingly slow however). Lindelof plays totally different with good awareness, reading the game etc. I personally believe that is why many rank Lindelof so low, because he has that quite style. He shuts down situations before they happen instead of recover with a tackle, with speed or with pure strength.
In the end, United need both of them, and also Maguire, to be sucessful. You must have at least three good centrebacks. There will be injuries, there will be two tough matches within four days, etc.