I think it's pretty obvious that our biggest need right now is a starting level RW. Of all the starting positions, we are weakest in that spot by a country mile. Sure, we could use another backup CB and backup CM, but to improve our starting line-up we need a RW. It doesn't really matter to me if we play either Messias or Alexis, because they're pretty much at the same level with different characteristics. Neither of them is good enough. Messias won't get much better due to his age and even though Alexis might improve, his ceiling is too low for our ambitions.
In midfield I don't see any room for another player if we don't let at least one or two players go. I think it's safe to assume that our starting midfield will be Tonali, Bennacer and de Ketelaere. That leaves Diaz, Krunic, Adli, Pobega and Bakayoko as bench players. More than enough numerically. There might be some room if Bakayoko leaves, but it would still mean that at least two midfielders won't see much of the pitch at all this season.
In defence we have room for a third/fourth CB and we should be looking for cheap talented player that can slowly grow with the squad. Let's see what Kjaer has left in the thank first, before we spend a lot of money in defence while we could use it better for the RW position.