Duffy was a key man in the Blades’ rise from League One into the top flight in the space of just three seasons.
But tragically, for Duffy and the club’s fans, the popular Liverpudlian never played in the Premier League after a contract dispute over the summer saw him loaned out twice while Chris Wilder’s men threatened to qualify for Europe.
Duffy then left United when his contract expired, but he remains an iconic figure at Bramall Lane – not least for his remarkable goal in the Sheffield derby at Hillsborough in 2017, which will go down in Blades folklore.
Now 36, Duffy recently signed semi-pro terms with Macclesfield to enable him to embark on a coaching career and opened up on his departure from Bramall Lane on the Blades’ official podcast.
“There’s always regrets,” Duffy admitted.
“Me and the gaffer have patched things up now and we’re on good terms. But both of us agreed that we could have done certain things differently.
“I only had one year left on my contract and I wanted to try and speak to the club and try to extend it by a year. That was it, really. Look, I think I caught the gaffer on a bad day. But once he’s made a decision, it’s very rare that he goes back on it.
“And I’m pretty stubborn myself; I didn’t really feel as if I had something to apologise for, because I just went in for a general chat with him and to see if I could extend my contract.”
In some quarters the situation was twisted to suggest that Duffy had demanded a pay-rise from United, with the player and his agent accused of greed. But Duffy insisted: “It didn’t even get to the stage of talking about money.
“I got thousands of messages saying your agent did this and whatever, but I had one year left on my deal and wanted to hopefully extend it by a year.
“Everyone else had one or two years and I just thought I’d go in for a general chat. It just escalated a little bit. I thought I’d leave it and let him calm down, and I ended up training with the group that wasn’t wanted.
“I was like: ‘That’s not right, having me train with people on the transfer list.’ But I left it and carried on my training, thinking it would blow over, and it didn’t.
“Stoke came in and the club said I could go. It was heartbreaking for me afterwards. When I went to Stoke, I was like: ‘This is not going to work. This is just not the same environment that I’d come from and the same group of people.’
“But the hardest part for me was the effect it had on my friends and my family. They’d been on that journey with me and everybody was so excited to get to the Premier League. It probably hit them as much as it hit me.
“I think I’m pretty strong but there were times when I was lying in bed thinking: ‘Why did I go in and speak to him that day?’ At the end of the day, if I didn’t go in on that day then it wouldn’t have happened.
“So part of the blame lies with me and that’s something I’ve got to live with for the rest of my life. But I’ve spoke to the gaffer since and I speak to him quite often and if there’s one thing both of us wish we could turn back time, it’s probably that day.”
Despite the sadness over his exit, Duffy insists he holds many fond memories of his time at United and still keeps tabs on his former club’s results and fortunes. He remains close with a number of his old teammates, most notably his friend and fellow Liverpool fan Jack O’Connell, and has appeared on United’s in-house SUTV show as a guest pundit.
“It was the best group of players that you could ever wish for,” Duffy added.
“Every single day on the training ground, it would be everyone laughing and joking but as soon as the boots were on, it was no messing about. Full throttle. Big tackles, big headers, big races.
“I remember the gaffer stopping the session one time. The ball dropped in slow motion, Jack and Flecky [John Fleck] were going in and I was thinking: ‘Oh my god!’
“Both of them, full throttle, went in and the ball burst. The gaffer stopped it and said: ‘Look lads, I love everything you do but this is going a little bit overboard. No one would pull out. A lot of new players would say: ‘Your training’s mental, how intense it is’.
“The sad thing for me is that a lot of people jump to the end of the story and want to know what happened at the end, when there’s so much good that happened,” he added.
“I could talk about all the good stuff for so long and it’s great that we’ve sorted everything out. It was just one of those things that can happen and unfortunately it did happen. It was a learning curve for me.”