Ambling around and not being creative are 2 different things. You suggested he wasn't being creative. Seeing as he has been more creative than Silva this season, I can't see your point.
My claim simply is that when we have both playing at the same time, Yaya isn't as mobile or aggressive defensively, while Silva is positionally undisciplined. The confluence of these 2 factorsvmakes defending clearly harder.
When one or the other isn't playing, it is easier to accommodate the defensive weakness, as it is not compounded by the presence of both players. An example is at Sevilla. With Sterling and Navas being positionaly congruent most of the game, Yaya's 'ambling' as you put it wasn't that much of an issue. Especially with 2 CMs also flanking him in the middle.
As for creativity, Yaya provides it constantly.even while ambling defensively. And on that front, he has been superior to Silva this season.