Although United manager David Moyes refused to discuss the 25-year-old following Wednesday's Capital One Cup semi-final loss to Sunderland, sources involved in the deal had confirmed it was close to being agreed.
The seal was put on the switch by Chelsea manager Mourinho when he spoke to the media on Friday lunchtime.
"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."
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