I don't think it should be too difficult for Luiz, playing alongside Mikel he should relish the extra freedom. Agreed, it will be difficult if we are expecting him to take over the holding role, that does require a specialist. How did they look together today ?..it's difficult to tell from the highlights, because we didn't appear to be under much pressure.
Ps..I actually like Mikel when he keeps it simple. He was growing into the role, it's only recently (for some unknown reason) that he's started venturing foward more. He's got great upper body strength, he stays on his feet and doesn't dive-in recklessly, shields the ball well, all qualities needed for that position. It's true, he's one-paced, and you are not going to see many defence-splitting passes from him. But that's true of many who have been successful holding players in the past.