The Brazilian midfielder has just completed a season-long loan switch to Atletico from Wolfsburg.
"I am glad to join Atletico, I was really eager to play here because this is a great club and I could not have arrived at a better moment," Diego said at his unveiling.
"I want to win every match I play. I hope I can win titles and that we can play in the Champions League next year.
"It is no secret this team have a lot of quality. There is no need to think about Real Madrid or Barcelona, we have to work thinking it will be 11 players against 11 on the pitch."
Diego made the move to Atletico on the transfer deadline day following a spat with coach Felix Magath towards the end of last season. Diego is in a race against time to be fit for the club's clash with Valencia on September 10.
"I have had a very hard physical preparation, I began doing triple training sessions per day on June 20," he said.
"But I have not played a match in three-and-a-half months and that sets a difference, though I feel fine physically. In the next nine days I will do everything to be fit."
The 26-year-old Brazil international started his career with Santos before moving to Europe with Porto in 2004. He joined Werder Bremen in 2006, then spent the 2009-10 season in Italy with Juventus before returning to Germany with Wolfsburg.