No one is arguing the merits of Holtby. I hope he does replace one of them, Siggy. Siggy back out on the left wing with Holtby and Eriksen sharing time in the No 10 role gives us two excellent players each in those respective positions, per AVB's own stated intent. I think we can all agree we'd prefer more cover at fullback, but with what we currently have in those positions we can still beat the better teams of the league. We will not beat those teams with our current situation at the CAM position.
If we play a three of Capoue-Paulinho-siggy we will. Those three are direct, quick at passing and the operate at different positions on the pitch giving us a really good central spine/link to our widemen/striker. Capoue will take the roll of collecting the ball from our CB's (mostly Dawson) and bring it out to anyone on the pitch that's in space really as he seems to have a cracking pass on him.(Capoue looks better offensively than Sandro, but pre-inj Sandro has him beat hands down defensively, and Sandro should considering he was the best DM in the world at that point.) Paulinho is the midrange/late arriver and will give support to Capoue and wingers and FB. Sig operates further forward and will support striker/wings and supply them with those sexy slid in behind passes he is capable of, not to mention once he's tweaked his sight a little bit, bang some curlers in. I'm not including dembele because of the current midfield selection we have he is the least direct and creative. He slows down our play because he has a compulsive need to take on a player and two before he passes.
Once Holtby is back from injury I expect him to oust Siggy as Holtby is just a class above most of our central players once he's playing to his abilities (Should see some of the stuff he did at Schalke and for the german U21's. In a word; fantastic.)
So in essence, we have two true defensive midfielders who when played together will shut down any team and still offer enough going forward to not hamper us too much, We have one real box to box CM who can score, pass and tackle, we have one quasi-CM who has a tendency to dribble too much before he passes but will be excellent to bring on if we need to slow everything down and just hold on to the ball. We have a dogged playmaker in Holtby who was in the top 5 assist in his last half season for Schalke in a league that boasts Schweini, Gøtze, Gyndogan, Castro, Reus, Draxler, Robben, Ribery, Kroos and the list is barely started. We have an up and coming pure playmaker in the shape of the Carroll who just oozes class whenever I've seen him on the ball. And then we have Siggy who mighty not be the flashiest CAM in the world but he get's things done and played many a good pass against palace and last season, he also has one hell of a shot once he can fine tune the sight on goal (a goal or two should see to this.).
For me we do not need any new CAM as we have three-four players who can play there and do an excellent job and I've not even mentioned Chadli as an option centrally... Should Bale stay as well all of the above will get more space to strut their stuff as he will be more closely watched than Chadli and Lennon currently are.