So let me offer my 2 cents as a real madrid fan who's actuallu watched james a lot these past 3 years. James is great. His workrate is great, IQ, skill, vision, finishing, he's even got deceptive pace(truly it's acceleration he lacks, not pace). He'd massively improve your team, provided mourinho doesn't shackle him to a flank but gives him enough freedom to cut in and pick his spots. A front 3 of james-zlatan-miki, with pogba linking up with them would be terrifying to behold. You'd have to switch permanently to a possession based style, because that's where they would thrive, and i'm not sure mourinho would be willing to do that.
He can't really play in midfield for you because you already have pogba there, and switching to a 4-2-3-1 likewise makes little sense.