If you're playing a 4-2-3-1 then both players in the pivot have to be able to link defence and attack. Gomes does that, Gueye doesn't, while Sigurdsson doesn't offer himself as an option between the lines as he doesn't have the skillset to receive the ball and assist in build-up. Therefore everyone marks Gomes out of the game, we're forced out to the wings or forced to go direct where the opposition can regain the ball more easily (Especially on the right where Coleman and Walcott are turnover machines). Silva has always said his preferred formation is 4-3-3, he doesn't have the suitable personnel to pull it off (Sigurdsson can't play in that system as, again, he doesn't have the skillset to do so and we don't have a natural like-for-like replacement there that's up to the required standard).
We need new players. and quite a few of them to fix our problems. Make no doubt about it