Diamond played in Sunderland’s opening-day win over Wigan Athletic at the Stadium of Light at the beginning of this season but was limited to appearances in the Carabao Cup after that.
He was then loaned to Harrogate Town in League Two having previously helped the club to promotion from the National League. Diamond enjoyed a fruitful season under Harrogate manager Simon Weaver, with the pair appreciating a good relationship.
He netted 13 times in League Two and provided a further six assists, taking his league goal contributions to an impressive 19 this campaign.
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“I would say Jack Diamond coming back is unlikely now, but we really did try,” Weaver told the Harrogate Advertiser.
“We had a good go at getting him back here again. We’ve tried to sign him on a permanent deal but got knocked back. We’ve tried to bring him back on a loan basis, on any basis we could to be honest.
“We've tried, but Sunderland own Jack, he is their player and we have to respect that.
"With regard to the loan, they've said to us that they want to see the lad tested at League One level now that they have gone up to the Championship.
"Obviously it is disappointing because Jack Diamond is a real talent and he made a big impression during his time here. But, we are excited by the transfer business that we have already done this summer and the other deals that we are still currently working on.”