The Super Lig transfer window closed on Friday evening.
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And the Manchester Evening News says Mather was in talks to join Kayserispor outright.
But United eventually pulled out of talks believing there wasn't enough time to close the deal satisfactorily.
Kayserispor wanted Mather in a permanent deal. A fee was settled with United, with a buy-back clause included.
The player had also verbally agreed contract terms. But the deal fell through as United management decided there wasn't time to close out the transfer.