United finally agreed terms with Lonergan three weeks after first beginning negotiations with the 27-year-old, and he was due to complete his transfer this afternoon.
Preston accepted an offer of around £200,000 from Leeds at the start of this month, but United suspended their interest in Lonergan after deciding that his initial wage demands were excessive.
The club’s attempt to replace Kasper Schmeichel – sold to Leicester City last month – continued with an approach to take David Stockdale on loan from Fulham, but Fulham’s insistence on a loan fee and a 24-hour recall clause saw United reconsider a deal with Lonergan.
The Preston-born keeper is an experienced player with more than 200 league appearances behind him, and he is set to rival Paul Rachubka for a starting place at Leeds next season.
Grayson revealed that a signing was imminent but refused to reveal the identity of his target, saying: “We’re 95 per cent sure of having a signing today.
“We agreed a deal (on Saturday) but until it’s been signed, sealed and delivered, I’m not going to say who it is, what position it is or say that it’s definitely done.
“The player’s agreed to a deal and we should have it finalised today. It’s someone who we think will influence the squad.”
United’s players were preparing to travel to Norway today for a pre-season friendly against Sandefjord, but their expected signing was unlikely to be confirmed in time for him to make the trip.