The 25-year-old midfielder's 37 million pound move to Old Trafford is now subject to a medical and agreeing personal terms.
The Premier League champions confirmed Friday evening that the deal is now subject to the midfielder completing a medical and agreeing personal terms.
The Spaniard was expected to arrive at Carrington on Thursday in order to complete his medical but Chelsea's negotiations with Basel over Mohamed Salah held up United's plans.
In order for Mata to make his debut against Cardiff City on Tuesday, United must register Mata by midday Monday.
Jose Mourinho revealed Friday afternoon that Mata was close to a move to Old Trafford and claimed the midfielder was keen to leave Stamford Bridge.
"I asked him when the offer came: 'Are you really happy to go?' He said 'I'm really happy to go,'" said the Chelsea manager. "He is a fantastic player. In spite of not playing much for us, I still think he could be important for us. It's a good move for Juan, it's good for the Premier League and the offer is the right offer."
Mata has struggled to secure a regular first-team place since Mourinho's return to Stamford Bridge, making just 18 appearances for Chelsea this season.