My guess is that when Gerrard tried to get Coutinho here, he promised him all the game time he would need at Villa ahead of the World Cup.
When we play a setup with two strikers, there’s only room for one south american nr 10 and so Buendia will usually be starting on the bench.
I believe SG really rates Buendia, but he may have painted himself into a corner somewhat regarding Coutinho.