My take is that if koulibaly and him sign then with koulibalys ability to read the game and move forward to get the ball he can be the defensive legs behind carvalho...also with both of them we will have 2 big lads who have great long distance passing to unleash lukaku and wingers making us a real danger for teams.
As others have said he needs to be paired with someone who can do the leg work but only if hes isolated which he wouldnt be with koulibaly stepping out.....id personally stick a playmaker alongside him and have both deep as then we wouldnt really need a box to box player with carvalho + koulibaly + a playmaker able to pass it wide or forwards....