For one reason only - balance. Against Palace we didn't use him and the side was well balanced. We created a fair few chances, we also didn't concede too many opportunities and certainly not too many clear cut opportunities like we did time and time against Everton for instance. With Yaya, Silva and De Bruyne we're not well balance for certain games. Like I said, long ball teams we can press their defence to stop them getting a decent ball forward and mop possession up. You don't need that extra man in there really.
Swansea will play a more "purist" style than this. They'll look to get the ball on the ground and find space. They're good enough to pass out from the back, it's been their way for many years. If we push up and they do manage to find space, as Sigurdsson in particular will look to, behind our midfield then we might have problems.
Spurs managed to score by sucking our defence out of position because we didn't have midfield cover. You can point the finger at our defenders (Otamendi in particular) but the fact is they felt drawn out because Yaya's trying to get back but isn't defensively covering well enough. We need to get in a habit of stopping conceding such "easy" opportunities for the opposition to get into goal scoring positions.
Having Fernandinho in there to cover the ground defensively will help us eliminate the potential to be torn open so easily. Yaya, Silva, Sterling, Sane and Jesus will create opportunities. De Bruyne is excellent and in certain games they can all work in the same team, but I'd leave him out of this one to start with. In other games I'd have him in ahead of Silva but I like Silva's ability to manage the game in terms of its pace. He dictates whether we want to make the opposition chase the ball for a ten minute spell, suck them out of position and get at them, or play direct and attack quickly. I feel like De Bruyne still needs to develop that control, he's always 100% geared on going forwards and trying to create a scoring opportunity. In doing so we can lose the ball and be exposed so Pep will want us to always be in control of the pace of the game. It's less tiring chasing the ball and when you have Silva and Yaya on the pitch they can do with playing at their own pace!