The 25-year-old made his Spain debut against Serbia earlier this year.
Garcia joined Benfica from Spanish giants Real Madrid in 2009.
Garcia will replace Nigel de Jong, who left City for AC Milan earlier on Friday.
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Summer 2016 is set to be a very exciting time for football fans, as this is when the long awaited Euro 2016 tournament kicks off.
Since Guus Hiddink arrived at the helm, Chelsea have improved considerably after a tricky start to the campaign.