McTominay struggled, as did most of his colleagues, as United were soundly beaten at the London Stadium. But the 21-year-old Scotland international can hardly be blamed for his part in the Red Devils' third Premier League loss of the season – he was set up to fail.
This was the second time Mourinho has elected to use a 3-5-2 shape – although, in actuality, it played out much more like a hyper-conservative 5-3-2 – this term, having flirted with the idea frequently in pre-season.
The first was against Tottenham Hotspur at Old Trafford in August. Then, as they did against West Ham, United shipped three goals, albeit without reply on that occasion.
Then, too, Mourinho fielded a midfielder as the right-sided centre-back; Ander Herrera occupied the role against Spurs and, as many would have predicted, the Spaniard was out of his depth and out of sync with his defensive colleagues.
It was strange, then, for Mourinho to repeat the tactic against West Ham. Especially given the Hammers, despite a recent credible draw with Chelsea, had struggled under new boss Manuel Pellegrini and hardly appeared to pose the kind of threat warranting of such a cautious approach.
Stranger still was Mourinho's decision not only to play a midfielder on the right of his back three again, but for that man to be McTominay, who, with just seven previous Premier League starts under his belt – and none this season – is hardly experienced in his primary role, let alone this alien one he was asked to fulfil.
Perhaps Mourinho's use of midfielders in his backline is a ploy to demonstrate how short of quality options he is at centre-back, having been denied a new signing in that department last summer. But Eric Bailly – a defender Mourinho signed for £30million just two years ago – was left an unused substitute, while Phil Jones didn't make the match-day squad.
Even Matteo Darmian, the Italian defender constantly linked with moves away from Old Trafford, would have made more sense in the position than McTominay. Instead, the young midfielder was, in many ways, hung out to dry.
McTominay is clearly a player Mourinho trusts. The 6ft 4ins Scot does, after all, fit the physical requirements of a typical Mourinho midfielder, while his safety-first approach in possession chimes with the manager's own philosophy. However, it seems McTominay has become a pawn in the former Chelsea and Real Madrid boss' politicking.
Mourinho named McTominay his Manager's Player of the Year at United's end-of-season gala in May, an award the 55-year-old boss created especially, he claimed, to recognise the youngster's contribution.
On the face of it, this was a commendable move, a boost of confidence for a young player making his start in senior football. Those with a more discerning – call it cynical if you must – eye, however, saw this as Mourinho polishing his own trophy, so to speak, by showing the world how he had adhered to United's proud tradition of putting faith in young academy graduates.
Although McTominay had produced a string of efficient and responsible displays at first-team level, he had played just 630 league minutes in 2017/18. The club's official Young Player of the Year gong went elsewhere and, even in a season of relative disappointment, there were several players whose contribution was greater than McTominay's. The award looked preposterous and raised eyebrows, but again, this was no fault of the player's.
There is also the example of Mourinho dropping star creative midfielder Paul Pogba in favour of McTominay for a crucial Champions League last-16 encounter with Sevilla last season. United were limply eliminated and the 21-year-old's inclusion perplexed and angered many fans.
McTominay has rarely disappointed in his senior appearances to date, yet likewise he has hardly inspired hope of better things to come. He is a solid player, reliable and sturdy. But he did not rise through the academy ranks with any great hype, such as, say, an Angel Gomes, an Adnan Januzaj or an Andreas Pereira – whom McTominay apparently outranks in the pecking order.
These limitations are not failings of McTominay's; someone of his profile makes for a useful presence in any well-rounded squad. But the profile he has been given by his manager is undue and unfair.
Marouane Fellaini, United's only signing of the first post-Ferguson transfer window, was for a long time seen as the poster boy for David Moyes' disastrous tenure at Old Trafford, having followed the former Everton boss from Goodison Park. Perhaps only now the Belgian is beginning to shake off his association with the long-departed manager.
McTominay, through no fault of his own, is in danger of becoming similarly indelibly linked with Mourinho's crumbling reign.