The striker sealed a move back to Atletico Madrid from the Premier League champions in September, but not before he went AWOL in his homeland of Brazil as his relationship with Blues boss Antonio Conte grew increasingly fractious and the Italian told him he could leave Stamford Bridge.
Costa has played just 14 games in all competitions this season due to only being registered by Atletico in January because of their transfer ban, which has resulted in Spain coach Julen Lopetegui leaving him out of his latest squad for friendlies against Germany and Argentina.
And the 29-year-old insists the fact he has played so little this season is not his fault, with the striker telling reporters: "It is very clear that it was not my fault that I was in Brazil.
"We all know what happened there but I’m back now and I want to do my best.
"The coach (Julen Lopetegui) was always very clear with me that if I was not playing then it would be impossible for me to be called up.
"But he also said that if I was playing well then the door would be open to me and I’m grateful to him for that."
Costa was replaced in the Chelsea first team by Alvaro Morata, who plays alongside the striker in the Spanish national side.
However, Morata's poor recent form at Chelsea - he scored his first goal of the year on Sunday - also meant he missed out on Lopetegui's squad.
But Costa is confident Morata will be back in the Spain squad soon, and admitted he relishes the competition that the 25-year-old gives him.
"I miss Morata because we get on well," Costa added.
"Competition is always good. Morata has been having a bad time there but he is back now scoring goals and I hope he comes back with us and we can play together."