In terms of why the Red Devils acceded to the transfer, one can only assume it's because the management team felt they had better or more reliable options in midfield at the time.
Still only 28 years of age, the Scot has years left in the game at both domestic and international level if he remains injury free.
At £25.7m, McTominay was a bargain for the Partnopei, but it wasn't an easy decision for him to move there after 22 years with the serial Premier League winners.
"Obviously it's a big decision, there's no denying that. It was a huge decision for me and my family and my friends," he said at the time.
"At times like that in your life you just have to say, 'Do I want to do it? Absolutely,' and go for it. There's no looking back. I've never had any regrets in my life or my career and I continue that way to this day.
"I want my career to be successful and I'm going to go out of my way to try and push myself to be the best football player and best person that I can possibly be.
"I've got great people around me who help me so much and the stress in my life is very minimal because I've got a really good family, amazing friends and I have a lovely girlfriend who (all) do so much for me.
"It's not easy going abroad and having to settle in but it's been quite seamless to be fair in terms of how much the Italian people have helped me settle into my house and whatever. Things I need, like a car or a taxi or anything like that. I'm very grateful to them."
McTominay's all-round figures with Napoli are impressive
With just three points separating Napoli from current Serie A leaders, Inter, it's worth delving into the details and seeing just how important McTominay has been for club and country this season, and whether, in hindsight, he made the right decision.
Of the 34 games played in all competitions for Napoli in 2024/25 he's started in 31, and nine goals gives him around a one-in-four ratio from midfield which is a healthy return.
That number is even more impressive when you take into account that his seven league goals are only bettered by Romelu Lukaku's 10 at the club.
Only four players at Napoli have played more than McTominay's 2,272 Serie A minutes, and given how combative the Scot is, earning just three yellow cards throughout the campaign so far is something of a personal triumph.
In terms of his passing, completing 592 from 699 total passes gives him a top 10 ranking of 84.7%, which is high enough in itself and perhaps goes to show just how much stock Antonio Conte holds in his player's abilities to keep hold of and dominate possession.
What won't be a surprise to anyone is that McTominay sits proudly atop the table for the amount of duels he has had with an opponent. 307 in total is 41 more than the next Napoli player, in this instance, Frank Anguissa.
81 aerial duels are the third most in the squad, whilst 108 ball recoveries places him fourth in that particular metric. 35 clearances and 20 tackles won from 36 shows what an all-rounder Man United gave away.
His importance to his club side is clearly therefore not in question, but how do his numbers for Scotland compare?
To begin with, he was one of only six players that started all six games in this season's Nations League - where they agonisingly missed out on progression to the quarter-finals by just a point - and of the seven goals they plundered McTominay top-scored with two, the same as John McGinn.
Two Scotland players also hit the woodwork during the group stage of the tournament - McTominay and Billy Gilmour.
The Napoli man was responsible for two of their four fast breaks in the competition, and his shooting accuracy of 66.7% was only bettered by McGinn - whose two goals from his two shots gave him a 100% statistic.
An 89.4% pass completion statistic is just below Scott McKenna's 93.2%, the latter completing 136 of his 146 passes compared to McTominay's 135 from 151.
As with his club output, the 28-year-old is right up there when it comes to getting involved, and his 53 total duels are only one less than Gilmour and Lyndon Dykes' 54.
Winning possession back on 26 separate occasions is the third-best effort of Steve Clarke's entire squad, and 12 of those positive encounters have come in midfield, just one less than Anthony Ralston.
10 fouls won and just one yellow card received again evidence that here is a player that has no issue whatsoever in getting stuck in, but not at all costs.
When the going gets tough, McTominay evidently gets going
McTominay is of no use to anyone if he's stuck on the sidelines and a maturity to his game at all levels could be the reason why he's managed to generally stay out of trouble for the most part.
Putting the above data into context shows what many Scotland supporters will probably know already... when the going gets tough, Scot McTominay gets going.
Whilst it wouldn't necessarily be fair to call him their all-action hero, there certainly isn't another player in the squad that can post the all-round numbers that he does.
Getting more regular match time with Napoli will undoubtedly have improved his consistency at both club and international level, and it does highlight again United's strange decision to move McTominay on rather than finding him a position in the first-team starting XI.
When all at the Theatre of Dreams were looking around for leaders amongst the squad - how they could do with another one now - United had one in the palm of their hands and allowed him to slip away.
To that end, Sir Jim Ratcliffe et al must be seething after seeing just what an impact McTominay has made since leaving the club, and if nothing else it should serve as another vital lesson.
