It was an extraordinary period to arrive at Old Trafford, but Rathbone had not been recruited to treat the first team or take charge of the club's rehabilitation programme. His appointment came about because of something rather different: a book about his own struggles with confidence as a professional footballer.
Rathbone had spent years trying to understand why talented players could be undermined by their own doubts. Having experienced those problems himself, he eventually wrote about them, only to discover that his honesty had opened an unexpected door into the world of elite football.
“So when my first book came out about me having no confidence as a player, incredibly, I became the guru of confidence," Rathbone told Football Presse. "How weird is that? I had no confidence. I'm now the expert on the field.
“My son was at Man United as a young player at the time. I knew Dr Steve McNally, the club doctor, from my time at Everton. He was at United when we played them. And the other guy was Brian McClair, the legend of the club.
“They read my book and they thought that they had an under-23 squad with the likes of Sam Johnstone the goalie and Jesse Lingard and Michael Keane and Andreas Pereira and Tom Lawrence. They're a fantastic bunch of young players, and they felt having somebody around who lived it, experienced it, written about it, suffered with it, could be a great help.”
The idea was to give Manchester United's young players access to someone who understood the psychological challenges of a football career from personal experience. Rathbone had played professionally, struggled with his own confidence and subsequently spent decades working in the game. He could speak to young footballers about the pressure they faced without pretending that he had always found it easy himself.
It was a role that brought him into the club's academy environment, where the players were trying to bridge the gap between being promising youngsters and becoming established first-team professionals.
The appointment also placed Rathbone inside a club preparing for a historic change. Ferguson had announced his retirement, bringing an end to a managerial reign that had lasted more than 26 years. Rathbone's first six months at United would be Ferguson's last six, and the physio soon discovered that the manager was very different from the intimidating figure he had imagined.
“Alex Ferguson has read my book. He knew I was coming. He's gone to the trouble of going through my book. And that was a powerful moment.
“And he was unbelievable because, Jacob, I thought, right, he's going to be some miserable Scottish sod. That's what I thought. I couldn't have been more wrong. He was friendly, charming, super knowledgeable, just a lovely guy.
“You know the old thing, he knows everyone. He knew everybody's name and he could hold court or he could sit and listen. He didn't want to be the centre of attention. He was absolutely incredible.”
For Rathbone, the discovery was not simply that Ferguson could be friendly away from the touchline. It was the manager's willingness to engage with people throughout the club, listen to their experiences and give them the space to speak. Rathbone had arrived expecting to meet a man who ruled through intimidation, but instead found someone whose authority did not depend on dominating every conversation.
That impression became particularly important when Rathbone witnessed Ferguson's final day at the training ground in 2013.
While the manager's farewell at Old Trafford would be remembered by millions, Rathbone was present for a different and more intimate moment: Ferguson walking over to the academy staff and young players, including Rathbone, Nicky Butt and Warren Joyce, to deliver a message about what it meant to represent Manchester United.
Rathbone still sounds astonished when recalling that he was there to witness it.
“And I was there on his last day and he came over to where me, Nicky, Buck, Warren and the young lads were. I said, ‘My God, how am I here?’ And he comes over, he's got the sermon on the mic. He goes, ‘Sit down, lad, sit down.’
“He goes, ‘Warren and Butty, they're hard on you, but that's for a reason. This is the heaviest shirt in the world, and to go over the bridge to them boys and put that shirt on, you need to be something special. And you can't be a wilting violet. You've got to be a man. You've got a big heart, and we have to make those people over here.’
“And I'm there, I'm thinking, wow, am I here? Like, unbelievable. So yeah, it was fantastic, that.”
The message was about more than football ability. Ferguson was explaining why the academy staff demanded so much from the youngsters and why making the step into the first team required something beyond technical quality. The young players were being prepared for the responsibility of wearing a shirt that carried enormous expectations, and Ferguson wanted them to understand that the demands placed on them were part of that preparation.
For Rathbone, who had joined the club specifically because of his experience with confidence and self-belief, the speech carried particular significance. He knew how difficult it could be for a talented player to cope with the psychological demands of professional football. Ferguson was addressing the same challenge from the other side: the club had to develop young players who could withstand those demands when their opportunity arrived.
The manager's final day at the training ground also revealed how much his departure meant to those around him.
“Yeah, were there a lot of tears on his last day?”
Rathbone's answer was brief, but emphatic.
“Yes, very much so. Yeah, it was an incredible experience.”
By the time Ferguson left, Rathbone had witnessed both the personal warmth of the manager and the standards he expected from everyone working at Manchester United. The experience challenged his preconceptions and gave him a first-hand look at the culture that had sustained Ferguson's long reign.
It also gave Rathbone a remarkable opening chapter to his time at Old Trafford. He had arrived to help young footballers navigate the doubts and anxieties that could hold them back. Instead, he found himself working in the final months of Ferguson's career and watching the departing manager impress upon the next generation exactly what it would take to follow in the footsteps of those who had gone before them.
The heaviest shirt in the world, Ferguson told the youngsters, required something special. Rathbone was there to help them discover whether they had it.
Mick Rathbone is author of "The Smell of Football III: One Hundred Days in the Dug Out" from Pitch Publishing - which is available here.
