The issue is not the midfield which is very solid. The problem is that at least 3 of them are either plagued by chronic injuries/injury prone/coming back from a serious injury. In the worst case scenario, Marchisio comes back and looks bad, Khedira gets hurt within the next month, and Asamoah gets hurt again, we're up ****s creek. Furthermore, then you lose Lemina to AFCON. You then have Pjanic, Hernanes, and Sturaro.
Best case Khedira stays healthy, Marchisio comes back without missing a beat and Asamoah stays healthy. You then have Marchisio, Pjanic, Khedira which is very solid, with decent depth in Sturaro, Hernanes, and Lemina.
Dropping Hernanes and adding another CM would be ideal, but it MIGHT not be necessary. I still think the best thing is to add an AM like James because he can play in a CM position too (you wouldn't get a guy like Ozil because he is way too soft defensively, but James actually tracks back). That way if you do have a healthy midfield, you can play him in his preferred CAM position, and if you do not, he can fill in as a CM.