Brought into the club to work with a talented group of young players, including Jesse Lingard, Michael Keane and Andreas Pereira, Rathbone was initially unsure what to expect from a role that was very different from his previous work as a physiotherapist.
The then former Everton physio needn't have worried. Warren Joyce, the Under-23 manager, had other ideas.
"Unbelievable. Mate, I tell you, I'm there on my first day. I'm on my first day at Man United. I don't know what to expect. I've never had a role at a club where I'm not the physio," Rathbone recalled to Football Presse.
"So I'm there and I go in on the first day and Warren Joyce, who I played with at Preston, who's the manager of the 23s, he says, 'Come on, Baz, get changed.' I thought, thank God for that.
"Because to do the job, you need to be inside it, out on the pitch with the players running, laughing, passing the ball, joining the boxes, doing the press-ups and that. So you want to be a big part of that squad."
The Federico Macheda nutmeg
Rathbone's introduction to the group was memorable enough before the running began.
Joining a possession exercise, he found himself on the outside of a training box, with the younger players clearly wondering what the older newcomer was doing among them.
When Jesse Lingard moved the ball towards him, Rathbone suddenly had an opportunity to make an impression.
"I'm on the outside and everyone's looking at this old guy and wondering what the hell's going on.
"And the ball gets shuffled to me by Jesse Lingard, and then the thing stops and everyone's waiting for me, this old frail guy.
"And Macheda closes me down, so I slip it through his legs back across the box. Everybody stops, everybody's laughing and that, because I've knocked Fede Macheda."
Federico Macheda, the Italian forward who had announced himself to Manchester United supporters with a dramatic winning goal against Aston Villa in 2009, had become the first victim of Rathbone's arrival.
But there was another challenge waiting.
Beating Shinji Kagawa in the race
The players moved to the side of the pitch for a series of demanding running exercises, with former United midfielder Nicky Butt making it clear that he wanted to see some competitive spirit.
"I tell this story in my second book on the Man United chapter. We go to the side of the pitch and we're going to do some hard running in pairs, and Nicky Butt says, 'I want to see a winner.'
"And the run is from the side touchline near 18 and back, near six and back, far six and back, far 18 and back, far line and back. And it's a race, and that's a tough run."
Then came an unexpected invitation from one of United's senior players.
Shinji Kagawa, the Japanese international who had been sidelined for six weeks, approached Joyce to ask whether he could do some additional fitness work.
Rathbone was about to become his opponent.
"Shinji Kagawa comes over and he says, 'Warren, I've been out for six weeks, can I top up fitness?' And Warren goes, 'You can run against Baz at the end.'
"So everyone's laughing. The races go on, I'm watching them and it's competitive.
"And then at the end me and Shinji come on. Lovely guy, I feel like bowing to him, because he's Japanese, like, you know."
The contest began, and Rathbone found himself going stride for stride with a player who had represented Japan at the highest level.
"Anyway, we start off and it seems like I've waited my whole life for this moment. So first line and back, second line and back, we're neck and neck. The lads are all cheering, 'Go on, Baz!' I was laughing and that.
"Third line and back, fourth line and back, now the last run. We get to the far touchline and I say in my book, 'I'm sorry, Shinji, you seem a lovely guy and you're undoubtedly a world-class player, but this is my day.'
"And I just changed gear and just left him for dead."
The result was greeted with exactly the sort of reaction Rathbone had hoped for.
"Everyone's on the floor laughing. He's laughing his head off as well."
The man who had read his book
There was still one more surprise waiting on Rathbone's first day.
As he headed inside, he encountered Ferguson alongside fitness coach Tony Strudwick. The United manager immediately demonstrated that he had done his homework on the club's new arrival.
"Sir Alex sees me and he opens the door for me because I make it. And he's with Tony Strudwick, the fitness coach.
"He goes, 'Mick, I'm just telling Strudders about your book, how you don't like fitness coaches,' which isn't the case.
"I think, God, 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."
Rathbone had arrived expecting to encounter a formidable, perhaps intimidating, figure. Instead, he found someone who was remarkably approachable.
"I thought 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 a man arriving at one of the biggest clubs in world football, it was quite an introduction: a nutmeg on Macheda, a sprinting victory over Kagawa and a personal welcome from Ferguson.
Not bad for a first day at the office.
Mick Rathbone is author of "The Smell of Football III: One Hundred Days in the Dug Out" from Pitch Publishing - which is available here.
