Securing the title looks a difficult task for the Red Devils but they, at the very least, have to make Pep Guardiola's side work hard in order to claim the top flight crown.
José Mourinho doesn't tend to splash the cash in the winter transfer window, instead preferring to sign big in the summer and give arrivals time to adjust to his methods.
However, the Portuguese has admitted this week that he could be willing to spend in January if the right player becomes available.
He said: “I'm not a big defender of the January market but there are players and situations that sometimes you have the chance to do and you shouldn't lose that chance.
“I don't like the January market just to buy (for the sake of buying). But if you can buy one you would buy in June, with the chance a few months before, then great.”
So who could the United boss be talking about? We have picked out five players United should be targeting in in the January transfer window, although they will face plenty of competition from other sides.
Atlético Madrid striker Griezmann was reportedly United's top target in the summer, with the club willing to meet his €100million release clause.
However, the Frenchman elected to remain with Los Colchoneros in light of the Spanish side's unsuccessful appeal against a transfer ban, admitting he could not consider leaving the club when they would be unable to replace him.
United‘s official line at the time was that, considering Zlatan Ibrahimović's long-term injury, they had switched their transfer plans and made a more conventional No.9 their top priority, hence the signing of Lukaku. But their denial of wanting Griezmann up until the point he decided to stay put fooled no one.
However, Atleti will be free to sign new players in January when their ban expires, and have already secured deals for Diego Costa and winger Vitolo. Could United resume their interest in Griezmann mid-season?
The former Real Sociedad attacker signed a new contract with Atlético after his United move broke down, reportedly increasing his release clause to €200million, until next summer when it will halve.
But with a gesture of good will, and considering they have their brand new Wanda Metropolitano stadium to pay for, there is a possibility that Atleti agree to let Griezmann go for a figure below his new buyout.
Besides, the 26-year-old, while still one of the club's top performers, has not been in peak form so far this term, scoring five league goals from 14 games. With Costa coming in, the time could be right for Atleti to cash in on their No.7. A summer deal remains much more likely, though, and United would have to fight off competition from Barcelona.
Celtic left-back Tierney has been outstanding since breaking into the first-team at Parkhead in 2014 as a teenager. Still only 20 and a full international with Scotland, the young defender appears to have every tool at his disposal to become one of the best in the world in his position.
Blessed with an elegance on the ball and outstanding technique when crossing from wide, Tierney also has the pace and athleticism demanded of modern full-backs. He is able to traverse the touchline for a full 90 minutes, and is solid defensively considering his tender years and relative inexperience.
He wouldn't come cheap, but if United want a long-term solution to a position that as been a headache for Mourinho since he took over at Old Trafford in 2016, Tierney would fit the bill perfectly.
Mourinho was in charge for all three of Özil's seasons at Real Madrid, in which the former Werder Bremen and Schalke creator is widely regarded to have produced the best and most consistent performances of his career.
Although he is often a frustrating figure at the Emirates, at times seeming disinterested and lacking motivation, the 28-year-old remains an incredibly creative player, even when out of form.
With Henrikh Mkhitaryan and Juan Mata, United already have good players in the No.10 and right flank positions Özil occupies, but it can be reasonably argued that the German is an upgrade on both, at least in technical terms, and Mourinho doesn't seem convinced about either of his current options.
As with Griezmann, a summer move is much more likely. But if United want to bring him in sooner, Arsenal could be convinced to sell the player in January rather than losing him for free at the end of the season.
The 25-year-old, who was wanted by Sir Alex Ferguson before joining the French club back in 2013, has featured in just 72 minutes of Ligue 1 action this season.
In his absence, Julian Draxler has emerged as the one PSG player thought to face an uncertain future who will still play a prominent role alongside Neymar, Kylian Mbappé and Edinson Cavani, leaving Moura and former United man Ángel Di María on the outside looking in.
Mourinho was open in his desire to add a winger to his squad over the summer, but the club's pursuit of Inter Milan‘s Ivan Perišić ultimately failed.
Moura, at his best wide on the right and possessing tremendous pace, could be a good option for the Red Devils. He's not the kind of traditional wide man who'll swing crosses in from the byline all day, but he would offer greater width in the role than Mata and would be an additional goal threat.
What's more, Moura would not necessarily have to start every game. His current situation at the Parc des Prices is such that, especially in a World Cup year, he needs more regular game time, but is hardly in the position to demand a key role at a top club.
It seemed at the time that the England international's departure from North London was imminent, but he has stayed at Spurs. During Rose's early-season injury absence absence, Ben Davies established himself as a more-than-adequate replacement at left-back.
If United would prefer an alternative to Tierney with Premier League experience for their problem position, Rose is the standout candidate. At 27 and fit again, he could come straight into the side and instantly improve Mourinho's team.
Dynamic in attack, Rose could transform the Red Devils in all phases of play. And, with Davies in such fine form, Spurs might be more open to selling the former Leeds United man than they were previously.