The Blues began the day looking to sign a new defensive midfielder amid injuries to N’Golo Kante and Ruben Loftus-Cheek, but were rebuffed in approaches for Edson Alvarez and Ibrahim Sangare.
A loan deal was quickly finalised with Juventus for Swiss international Zakaria as the 11pm deadline loomed.
Standard Sport understand the loan switch can be made permanent for £26million during the coming season.
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On Thursday evening, Zakaria was filmed entering Juventus’ medical facility in the pouring rain to undergo his medical.
GoalItalia correspondent Romeo Agresti caught the player heading into the training ground with there no time for him to fly to England to seal his quickfire loan switch.
TuttoMercatoWeb then spotted Zakaria leaving 20 minutes later after the conclusion of his medical.
Chelsea are also hoping to complete the signing of Pierre-Emerick Aubameyang on deadline day after he arrived in England, while Billy Gilmour is nearing a move to Brighton.