United have been unable to sign off the transfer as quickly as initially intended due to the red tape involved in Chelsea's capture of Basle midfielder Mohamed Salah.
As they would have preferred the deal to be completed before sending Mata for a medical, the Spain star has been unable to link up with the United squad.
Now the club must decide whether to change that stance and instead send Mata for a medical over the weekend, which would allow him to train on Sunday and Monday ahead of Tuesday's Cardiff encounter.
Even with the temporary delay, United still see no reason why Mata would not be involved against Ole Gunnar Solskjaer's men, as the long process of trying to recover their status begins.
Speaking on Friday lunchtime, Chelsea manager Jose Mourinho suggested he was flexible in his approach to the transfer, which means United should not encounter a problem if they opt to give Mata his medical and get him on the training ground before the formalities were complete.
"It's what he wants," said Mourinho.
"It's what he asked us to accept. Chelsea Football Club is proud to act this way, in an open way, in a human way.
"The offer, compared with what the club paid two years ago (when Mata signed from Valencia), was fantastic."
United boss David Moyes will hope Mata's arrival will galvanise a team that endured its lowest moment on Wednesday when they failed to navigate a Capital One Cup semi-final against Sunderland, despite taking a first-half lead.
That they then lost what many claim to be the worst penalty shoot-out ever only increased the air of gloom.
It is patently obvious however, that Mata's eventual arrival can only be the start of a lengthy rebuilding process, with former United skipper Bryan Robson acknowledging bolstering central midfield - from which Michael Carrick is now likely to be absent with an ankle injury sustained in midweek - is now a priority.
"It is what everybody's been talking about for a couple of seasons now," Robson told Sky Sports News.
"Michael Carrick has been very consistent but everybody else has struggled to stamp their authority in there.
"I still feel it's an area we need to strengthen but it's only David Moyes and his staff who can make those decisions.
"But new signings, especially when they're top class, can give an injection of enthusiasm within the squad.
"It's a new challenge for Mata and I'm sure if he does sign, he'll enjoy being in the team and being wanted."
Meanwhile, Robson has rejected criticism of Sir Alex Ferguson's attendance at games.
Ferguson has been a regular presence at matches since recovering from hip surgery.
Whilst Moyes has insisted it is not a problem, others feel Ferguson is increasing pressure on his fellow Scot by providing a visual reminder of the success United have had over the past two decades.
Robson, who sat with Ferguson and Sir Bobby Charlton for the first-leg defeat at Sunderland, cannot understand the argument.
"It's a load of rubbish," said Robson.
"To see Sir Alex at matches shows that he's got great support for the club.
"For anybody trying to make an excuse and say 'Sir Alex is in the stand watching me play. I'm going to wilt' - well why didn't you wilt when he was manager?
"It's just weak excuses that the media and some ex-players and managers have come out with. It's a pathetic excuse.
"If Sir Alex wasn't going to games and wasn't giving David Moyes support, it would show he doesn't care, is selfish and is getting on with his own life.
"Instead, it shows he does want to see the club do well."