When he took over, the club had not finished in the top 3 in La Liga since the 2001-02 season and had not reached the knockout stages of UEFA Champions League since 1997. Diego Simeone has changed all of that emphatically.
Under Simeone, Atletico have won 5 trophies and have become one of the major threats in European football. He has guided them to a UEFA Europa League win. Most strikingly, they have broken the duopoly of Barcelona and Real Madrid to win the La Liga title in the 2013-14 season, becoming the first team in a decade, apart from the two heavyweights to do so.
Simeone is a tactical genius who has helped Atletico Madrid to glory with his great understanding of the game and the ability to motivate players. After a dismal show in Europe for more than a decade, he has guided them to two UEFA Champions League finals, beating teams like Barcelona and Bayern Munich in the process.
He has made them competitive again in the Madrid derby, after years of struggle against Real Madrid. Simeone has given Atletico Madrid a distinct identity which it lacked in years prior to him.
Diego Simeone has been successful in changing the way football fans world-over saw Atletico Madrid. Atletico are one of the most consistent teams in Europe at the moment, and when you consider what they have achieved in the context of the resources they have in comparison to that of other big teams around it is a massive achievement.
Atletico operate on a fraction of transfer and wage budget as compared to the clubs like Barcelona and Real Madrid, but they are the only team which has consistently threatened their superiority, both, in domestic, as well as European competitions in recent years, and a large part of the credit goes to Diego Simeone.
Atletico Madrid are not a club that will attract the top talents like Real Madrid, Barcelona or Manchester United and they also find it hard to keep their best players with them, over the years players like Falcao, Costa, Turan have left them to join other clubs and yet they have performed consistently - something which clubs with resources like United and Chelsea haven't been able to achieve.
It is because Simeone has been able to stamp his authority on the team and make them perform as a single unit irrespective of the personnel. He has made them one of the best defensive units in Europe. The way they complement each other on the pitch, work together to defend and their transition into attacks from defense show the tactical mastery of Simeone.
Atletico, now, are a very tough side to beat. Before Simeone, Atletico were not even considered as formidable opponents. But, that all changed since Simeone took charge. He has instilled an unbreakable team spirit into the team.
He knows how to inspire players and make the best possible use of talent at his disposal. Atletico, in terms of the talent they have, are not quite at the level of teams like Barcelona or Real Madrid, but it is the attitude that the team has developed under Simeone which has helped them mount a challenge domestically and in Europe.
What lies ahead....
The combination of this spirit and the tactical nous of Simeone has helped Atletico win every major trophy they have competed in, in last half a decade apart from the Champions League.
In the Champions League, their local rivals - Real Madrid have proven to be a barrier which they have found impossible to break. It would be great if Simeone can finally get a UCL title under his belt to cement his already great legacy at the club.
They have moved into a new stadium and they have probably built their strongest squad in recent memory. That combined with a great team spirit and tactical acumen of Simeone can finally help them win the Europe's most elite club competition.
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It will not be easy, but you never know! El Cholo might just have a final trick up his sleeve to propel them to the UCL title, which would be the ultimate crowning jewel for the Simeone era at the club.
Simeone has signed a new 2-year extension, keeping him at the club till 2020. Whether he wins the Champions League or not, his tenure at Atletico Madrid will always be considered as a turning point in the fortunes of the club.