Xabi Alonso decides Chelsea's 'final piece of the puzzle' with £51m release clause set to be triggered

Chelsea are preparing to make their final marquee signing of the summer after head coach Xabi Alonso identified one of the world's top goalkeepers as the so-called 'final piece of the puzzle'.

It has been an active transfer window for those behind the scenes at Stamford Bridge, with a host of new additions bringing their total spend to around the £300million mark.

Morgan Rogers became the most expensive British player in history after signing from Aston Villa for £117 million, while Maxence Lacroix (£52m), Marco Palestra (£43m) and Geovany Quenda (£40m) were also brought in for big money alongside Valentin Barco (£34m).

At the start of the window, the average age of Chelsea's squad was just 23.6-years-old. They didn't have a single player over the age of 30, but that figure has changed following the arrivals of Jordan Henderson and Danny Welbeck.

And it appears they are set to add another experienced player to their ranks, with FC Porto goalkeeper Diogo Costa being strongly linked with a move to the Blues.

Costa featured for Portugal at this summer's World Cup. Image credit: Getty

Costa, who has made over 250 appearances for Porto, as well as 48 caps with Portugal, has been identified by Xabi Alonso as the 'final piece of the puzzle', according to Spanish publication Mundo Deportivo.

In fact, Chelsea are prepared to trigger the 26-year-old's €60million (£51m) release clause ahead of the upcoming campaign.

Robert Sanchez established himself as the club's number one last season but for some time, the goalkeeper position at Stamford Bridge has been viewed as a weak spot, so the addition of Costa would certainly be welcomed by many.

Chelsea 'open to offers' for signing who hasn't played a single minute for the club

Chelsea's striking options are inflated, to say the least. Following the arrival of Danny Welbeck, they have Nicolas Jackson, Liam Delap and Emmanuel Emegha on their books alongside Joao Pedro.

Emegha officially joined Chelsea at the beginning of July, with the club having agreed a £22 million deal to sign him from Strasbourg back in September.

However, Fabrizio Romano claims there is a 'possibility' that the 23-year-old will leave if a 'good proposal' comes their way.

Romano also claims Chelsea are 'open' to selling Liam Delap should an acceptable bid be made, although the former Ipswich Town striker wants to stay at the club and fight for his place.