United announced the signing of John Obi Mikel on their official website, after a £4 million deal was struck with the Nigerian and his then club Lyn Oslo.
Meanwhile, Chelsea claimed to have reached an agreement with Mikel's agents and ultimately landed him after paying United £12 million in compensation, with another £4 million going to the 29-year-old's old side.
But Mikel could finally become a United player in January, with the Sun reporting that Mourinho wants his former Chelsea lynchpin to free up £89 million summer recruit, Paul Pogba.
Mikel, who is yet to feature for the Blues under new boss Antonio Conte, is out of contract in June and the Italian is said to have 'no plans' to renew his deal.
United have been tipped to shed Bastian Schweinsteiger from their ranks when the transfer window reopens, while another midfielder, Michael Carrick, is set to enter the final seven months of his deal.