McTominay appeared to favour his knee early on at Old Trafford, following a collision with Magpies goalscorer midfielder Matty Longstaff. He was substituted for Paul Pogba at the break, and Solskjaer offered a bleak analysis, per Samuel Luckhurst of the Manchester Evening News:
"He's done his ligament in the knee, probably. I don't know how bad it is, so let's see. The boy has got the biggest heart of the lot, he played until half-time and you stiffen up after. We might have to find out tomorrow to see how it is. When they knocked together but you see the attitude of the kid and everyone, to be fair, in the beginning of the game. Not sure if he's gonna be ready for Saturday (against Burnley), probably not, but we've seen him heal quickly before."
While Solskjaer praised how long McTominay was able to play on, the length of time the 23-year-old carried the injury is cause for concern. Luckhurst noted how the challenge that led to the problem occurred just two minutes into the game.
So McTominay played nearly an entire half with a weakened knee. Unsurprisingly, the Scotland international showed obvious signs of the potential seriousness of the injury, according to ESPN FC's Rob Dawson:
McTominay's combative style is always likely to leave him as much at risk as his opponents. He is a true terrier in midfield, one who hunts and scraps for the ball with maximum intensity.
His aggression was showcased in less than 30 seconds at Old Trafford when McTominay felled Sean Longstaff:
The reckless moment gave McTominay an unwanted distinction in this season's competition:
United need McTominay's snarling drive at the heart of the engine room. Solskajer's men are a different group without the protection McTominay affords the back four, as well as the energy he has to provide a man over in attacking areas.
Even though Pogba looked good on his return, and Juan Mata produced a 23-minute cameo, neither has the resolve nor the aggression to do the vital work McTominay performs. It's a similar story for Andreas Pereira and Fred.
Solskjaer needs a natural ball-winner to anchor the spine of his team. Nemanja Matic has those skills, but the 31-year-old has made a mere two starts and one appearance as a substitute in England's top flight so far this season.
Matic recently offered a coy response when asked about the possibility of reuniting with former manager Jose Mourinho at Tottenham Hotspur, per Omnisport (h/t Luke Gardener of the Daily Express): "As I know, you never know what's going to happen in life. At the moment I'm a Manchester United player and very focused on my job and in the future we will see what’s going to happen."
United will struggle to replace McTominay's rugged attributes if he's out for an extended period. Without him, there could be a soft underbelly at the heart of the team, one able to do lasting harm to the club's hopes of securing a top-four finish.