Atsu has signed a five-year contract and joins the Blues for a reported £3.5million fee.
Atsu has been loaned out to Dutch side Vitesse Arnhem for the rest of the season.
Atsu made 17 appearances for Porto in the league last season.
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The start of the 2017-18 Premier League season is just around the corner and is shaping up to be one of the most competitive yet.
As always it is the so called “big clubs” that have splashed the most cash in the transfer window so far this summer. Manchester United or City? Arsenal or Tottenham? Who are the winners and losers of the transfer window so far?