The Brazilian has been on the list of Atletico for some years, & now has the best opportunity to sign the player after his current Club does not count with the midfielder in its future plans & wishes to earn themselves his high salary. The Spanish Football Club needs a creative midfield player to organize the midfield & "feed" the forwards & Diego could be that player. The Brazilian has fallen in form & play this last season (injury) & is not the "eye catching" player of the past, but Atletico & Spanish Football could be the platform to re-launch his career.
Both Clubs are negotiating & the player has supposedly accepted a salary reduction to arrive at Atletico. Lets wait & see what happens in the next days.
By the way, other names sounding for Atletico are Rossi (Villareal), Falcao (Oporto) & Osvaldo (RCD Espanyol). Obviously theses players cost money & would come with the sale of Sergio Aguero. Stay tuned!