Atetico and Chelsea have announced a deal has been struck for the Spain striker.
Luis stated: "Proud of the effort to bring one of the three best strikers in the world.
"He comes here to fight, work and make us all better. I very happy that my friend is coming back."
Koke was also delighted with the arrival of Costa and spoke about his importance on the team.
"We are very happy, we know what he has done in the past for us and he is a very important man in football," the midfielder indicated.
"He knows there is competition and he has to win his position, and that is good for the competition and for the group."