Sergio Aguero could have formed a superstar with Didier Drogba at Chelsea – but John Terry had vetoed the signing.
The Blues were looking for a frontman in the summer of 2009 to bolster their aging attack – with Talisman Drogba and last season's top-scorer, Nicolas Anelka, over 30 years old.
Atletico Madrid striker Aguero seemed to be fed up after scoring 19 and 17 goals in his previous two La Liga games.
The West London team inquired about the promising Argentinian in the summer of 2009, but was deterred by the 40 million pound prize, according to Athletic.
Then Chelsea and Atleti faced each other in the group stage of the Champions League.
Aguero were defeated 4-0 in the Stamford Bridge match, but Aguero left the bench in the second half of the season to secure a duel, including a last-minute equalizer.
That did not impress Blues captain Terry, who had a big impact on the club and wanted David Villa to be the ultimate goal.
Terry said at the time, "I think we'll see Chelsea back and fight for the top players out there at the moment." People like Franck Ribery, David Villa. We want to attract this kind of players.
"Valencia is struggling for money, which is well known in terms of Villa, and there is much talk about Ribéry, we will be there for the players in the summer, but when big players like them become available, we have to show people that Chelsea is back.
"There have to be some changes, some players have to be brought in and a few who have not played in recent years have moved on and as players we said that our feelings about what we consider necessary will be improved Hopefully Roman will listen and things will be done. "
However, Chelsea manager Carlo Ancelotti was impressed and said, "I like him. I think Aguero can definitely play with Drogba. "
As a result, the blues increased their interest before the transfer window in January 2010, where they would bid £ 30m.
Atleti declined the offer, which was £ 10m off their bid price, and Chelsea wanted to avoid paying Aguero's Agent Bruno Satin a £ 5m fee.
Chelsea signed Daniel Sturridge, while Aguero spent another season in Madrid before moving to City.
Since then, Aguero has been in Manchester, scoring 244 goals in 353 games. He won three medals as Premier League winner.