The Spaniard, 27, joined the Blues last summer but rarely featured in the Premier League, making just 10 appearances.
Thomas Tuchel signed Niguez to bolster his squad ahead of a potential title challenge last year. However, a tilt to compete with Liverpool and Manchester City failed to materialise.
According to Marca, Niguez has already gone back to his parent club Atletico Madrid following the end of the season.
His departure leaves Chelsea short in midfield with N’Golo Kante missing large parts of last season through injury, while Billy Gilmour’s introduction to the first-team could be put on the backburner after a disappointing spell at Norwich City.
It’s reported new owner Todd Boehly will back Tuchel in the transfer market with a considerable war-chest but the delay in his takeover appears to have set back Chelsea’s summer plans.
Declan Rice is believed to be wanted by the Blues, but West Ham’s valuation of over £100million means a deal is unlikely to happen.
Meanwhile, another target – France and Monaco midfielder Aurelien Tchouameni – is thought to be close to joining Real Madrid.
Besides midfield, there are reinforcements needed in defence with Andreas Christensen and Antonio Rudiger both leaving Stamford Bridge.
Struggles in front of goal last season have also left Tuchel wanting new forwards this summer.
Speaking towards the end of last season, the German said: ‘If we stick to the group then we will try to solve it with these players. Is it necessary to bring more offensive players around the box to create more distractions, to create more space for each other?
‘We will ask that question. Can we do better in the structure? Can we push the players more? Did we rotate too much? Let’s see. There are a lot of options.
‘The numbers we produce from our offensive players don’t allow us to over-perform and we need over-performance if we want to be nearer to the top two teams.
‘I am not pointing fingers, I am not blaming anybody, but we need over-performance from everybody. It’s not over-performance if we have only single figures in scoring and assisting.’
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