We don't need replacements for either of Maddison or Sarr imo, but I do think quality competitions and additions could and would be useful. If you look at Maddison since his return he hasn't really hit the levels he touching before the Chelsea game. It's at times like this when your option becomes important, we don't really have that or at least Ange isn't using Bentancur or GLC in that way (which I agree with tbh). Likewise with Sarr, if he's not performing (which has rarely been the case tbf), you want to have a high quality alternative to come off the bench to change things up.
We do have some options and you could argue that between Bentancur, Donley, Bergvall, Donley, Devine and GLC (if stays) we do have options in that regard but the majority of them are unproven and we don't know if they are actually of the level we want and need.
It will be on Ange and the rest of the coaching team to evaluate the squad and determine what additions will be needed I. The more progressive areas of midfield but what we can't do is just think that what we have now is more than enough. The evidence has shown it's not.
DM is a lot more straightforward for me. We do need a new starting DM and Bissouma can be the competition/rotational option.