Assuming he'll join us, James will have to improve his work rate ~ doing the defensive work, which under Mourinho I'm quite confident he will. Going here, he would want to do well, to prove to his doubters wrong of his time in Real, and to also work well under Jose, I'm sure he'll listen and do the defensive duties well... or faced being dropped mercilessly and risk getting the label 'flop'.
As we can see from Pogba and Martial, two best recent examples, Jose instill the need to put more effort into the players, and they respond well, despite perhaps the early 'reluctances'. Having an already good attacking weapons in his arsenal but without the intent to mark and tackles, Pogba changed his playstyle to add the defensive duties so much that he's becoming a complete b2b midfielder. Martial at least mitigate for losing the ball too much with more defensive work to get it back.
Once James adapted to the system by improving his workrate, I think it could work in Jose's favored formation 4-2-3-1, and 4-3-3, the formation which Pogba seems to be at his best. Against lower teams, he and Pogba in a midfield of three would focus more on attacking yet supporting defensively the CDM, while against top teams, Jose could play him on the right wing with constant movement towards his more preferred center (like what Lingard, Mkhi and Mata often do this season) and letting the right back (Valencia) to overlap.
If James is joining us. I rather if Andreas Pereira to be given chances. Well, personally I want both, to add to the quality and competition of our attackers unit (Martial, Mata, Lingard, Mkhitaryan). At the moment, they're too inconsistent (all except Mata) and not providing enough the necessary goals (Mkhitaryan and Lingard).