The Athletic Bilbao midfielder emerged as a United target towards the end of the summer window, with a £26m rejected by the La Liga side.
He reportedly has a £31m release clause in his Bilbao contract, with conflicting reports suggesting United had triggered it on deadline day.
Confusion reigned as United 'representatives' were reportedly in attendance at the La Liga head office to ratify the transfer - but they were later revealed to be impostors.
But the farcical nature of United's transfer business has not put the club off preparing to return for Herrera in four months time.
And the report claims bookmakers expect the move, having cut the odds on Herrera to United in January to 11/4.
The Spanish playmaker was one of several targets for David Moyes in the final few days of the transfer window. The Old Trafford club reportedly made bids for Real Madrid's Sami Khedira and Cesc Fabregas over the summer.
In the end, Moyes settled for Marouane Fellaini, signing the Belgian international for £27.5m with minutes to spare on deadline day.
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