SAFC and League One transfer news: Coach gives insight into deadline day signing as Bolton's Keith Hill eyes 10th signing
Thirlwell, who is known to Black Cats fans having played for the club between 1997 and 2004, used his contacts on Wearside to seal a move for the 19-year-old on deadline day.
Indeed, the 40-year-old is a former teammate of Kevin Ball, whom he revealed he had spoken to in the build-up to Diamond's switch to North Yorkshire.
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Hide AdThirlwell attended the under-23s' 2-1 defeat to Aston Villa at the Stadium of Light on Friday - which prompted him to jump at the chance of signing the 19-year-old.
"I have spoken to Kevin Ball regularly since I left the club regarding their under-23 players," said Thirlwell, who also featured for Sheffield United and Carlisle United.
"I got a text last week saying that Jack might be available and he's a lad with a lot of potential so we went and had a look at him in a game against Aston Villa on Friday night.
"To be fair, he really stood out and he will add something a bit different to what we already have in the building.
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Hide Ad"He's got two good feet and he's in the squad to help us create and score goals.
"He is a lad who I know Sunderland think very highly of."
While the Black Cats concluded their business yesterday after the transfer window slammed shut, reports suggest THREE League One rivals might not be finished in their search for new recruits.
According to the Daily Express, Portsmouth, Bolton Wanderers and Fleetwood Town hold an interest in Graham Dorrans with his move to Kilmarnock hitting a stumbling block.
The former West Brom and Norwich City had his contract terminated by Rangers yesterday after two years at Ibrox.
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Hide AdElsewhere in the third-tier, Oxford United and Tony McMahon have parted company with the defender yet to feature for The U's this campaign.
And according to the Daily Mirror, Lincoln City manager Danny Cowley is now top of Huddersfield Town's managerial wishlist after being knocked back by Lee Bowyer.