The Athletic Bilbao midfielder was met by Sir Bobby Charlton with a reception party at Carrington after he arrived in a white VW transporter before being shown around, along with his agent.
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A club doctor is on site, though no medical has yet begun, as a 35 million euro transfer nears completion, with the Red Devils eager to do business quickly as his release clause would rise to around 40m euro on July 1.
Herrera is believed to have arrived in the United Kingdom on Tuesday and stayed in London before making the journey up to the northwest, accompanied by his agent and friends. Much of the legal work to push the transfer through was completed at United's office in Mayfair on Tuesday.
The midfielder's agent was cautious when questioned about the possibility of securing a move but told the press that "things were moving along nicely".
The party flew into Northolt on a private flight Tuesday ahead of talks with United officials at the training base on Wednesday.
Herrera would be Louis van Gaal's first summer signing and would see the 24-year-old finally complete a move that stalled at the end of the summer 2013 transfer window under then-boss David Moyes.
He said goodbye to his Athletic teammates before flying out from Spain, with United having offered a contract worth 180,000 euro per week.