Slater was expected to be unveiled at United’s Yorkshire rivals on Tuesday after Tigers boss Grant McCann promised the Sheffield-born youngster regular football, which would not be forthcoming at Bramall Lane.
Slater and his representatives hoped to seal a deal well ahead of tonight’s 11pm transfer deadline, after McCann and his employers had worked throughout the summer to put it in place.
Slater is understood to have turned down moves to other clubs in favour of returning on a permanent basis to Hull, where he spent time last season and won promotion to the Championship by winning the League One title.
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United even structured the deal in a way that would help Hull get around their inability to pay transfer fees for players, because they took a loan from the EFL last season.
The Star understands that the issue arose from Hull’s end of the transfer rather than United’s as they tried to renegotiate the terms of the proposed deal, meaning that Slater remained at United on deadline day.
And to add insult to injury, the late arrivals of Conor Hourihane and Morgan Gibbs-White in the last two days before the window slammed shut pushed him further down the pecking order at Bramall Lane. Slater now cannot move until January at the earliest.