The Spanish midfielder is expected to become Louis van Gaal’s first signing after the new United manager sanctioned the move to pay his buy-out clause.
That figure will rise to £34m on July 1 if the deal is not completed this month.
David Moyes wanted to sign the 24-year-old last August but the botched transfer descended into farce amid reports of imposters claiming to be United officials negotiating deals.
Herrera has been at Athletic Bilbao since 2011 and last season helped the club qualify for the Champions League.
Previously he was at Real Zaragoza and has played for Spain Under-20s, Under-21s and Under-23s, but not yet for the full national team.
He can play as a No 10 behind the main striker or as a centre midfielder, a position at which United have had limited options for some time.