A source close to the deal confirmed to Press Association Sport that it was "close" to being completed, with Mata expected to be in David Moyes' squad for the visit of Cardiff to Old Trafford on January 28.
Moyes refused to discuss Mata following Wednesday's Capital One Cup exit at the hands of Sunderland.
However, it is believed the 25-year-old has told Chelsea officials he will not be returning to the club, with the fee exceeding the ?30.75million Sir Alex Ferguson paid for Dimitar Berbatov in 2008.
United's morale could certainly do with a lift too after enduring yet another of those punches Moyes said he expected to take during his debut season.
And the Scot admitted the latest was the worst yet as his team contrived to gift Phil Bardsley a goal a minute from the end of extra-time when it seemed they were about to book a final meeting against Manchester City at Wembley on March 2.
Amazingly, Javier Hernandez responded almost immediately to take the tie to penalties, only for all United's takers to fail, with the exception of Darren Fletcher, as Sunderland advanced instead.
"We didn't play well enough," said Moyes.
"If we were (to go through), we were going to stumble over the line.
"With a minute or so to go we were almost there. We had opportunities to score a second goal and put ourselves in a better position but there is always going to be a loser in the semi-final and it was us."
Asked whether it was the most severe blow he had taken during his short tenure, Moyes said: "Yeah, I think it is part and parcel of the game."
It was tempting to think Mata might have been a useful presence in the penalty shoot-out, with Danny Welbeck, Adnan Januzaj and Phil Jones joining Rafael in failing to convert.
Moyes revealed Michael Carrick would almost certainly have taken a kick had he not succumbed to an ankle injury, whilst Hernandez - who has taken penalties for United before - was also ruled out after hurting himself celebrating his goal.
"Ideally, it wouldn't have been the group we had chosen," said Moyes.
"Javier would have taken a penalty but was injured and people like Michael Carrick would have been involved."
Source : PA