Who's the captain?
Michael Carrick left the wrong Portuguese on the bench at Chelsea, where Bruno Fernandes did not lead by example with another sloppy performance.
Fernandes, out of form for pushing three months now, was an incongruous choice to inherit the armband from Harry Maguire, supporting his teammates from the row behind the away dugout at Stamford Bridge.
Maguire's leadership has come under scrutiny this season amid his dozy defending and he was prematurely promoted by Ole Gunnar Solskjaer in his first season. Ralf Rangnick may not wish to upset the dressing room mood mid-season but many supporters would support him if he reassigned the captaincy.
In Rangnick's last season as a coach in 2018-19, RB Leipzig often favoured a 4-4-2 formation with a 'double six' - ie. two sitting midfielders. It seems inevitable United will adopt a midfield axis when they are lacking an athletic defensive midfielder and their sole specialist, the 33-year-old Nemanja Matic, is unlikely to meet Rangnick's counter-pressing demands.
Thomas Tuchel, one of Rangnick's pupils, is a staunch advocate of the system even though he is blessed with the world's standout defensive midfielder in N'Golo Kante. Tuchel believes one holding midfielder is left with too much ground to cover and Rangnick has willing runners in Scott McTominay, Fred and Donny van de Beek.
McTominay and Fred, an unfairly derided partnership that was overplayed by Solskjaer, still have a key role to play. Van de Beek could be the biggest beneficiary as Rangnick will demand a presence to supplement the attack and the Dutchman thrived from deep in the second-half at Watford.
Timo Werner and Yusuf Poulsen were a lethal front two for Leipzig and it would be illogical of Rangnick to ignore Cristiano Ronaldo's potency when he has scored more than twice as many goals as United's second-highest scorers.
Which big names miss out?
Fernandes has hit a ceiling at United and, amid all the chatter about Rangnick's approach jarring with Ronaldo, Fernandes could be in greater jeopardy if there is no room for a No.10.
Rangnick has emphasised how imperative aggression and intensity are to not just regaining the ball but influencing a counter-attack, which augurs well for Fernandes. Rangnick worshipped Johan Cruyff in his youth and, while controlling games is beyond Fernandes, his creativity inspired a myriad of wins during Solskjaer's tenure.
He may have to become more flexible under Rangnick. Fernandes showed promise in a slightly withdrawn role at Tottenham, where he exhibited his vision for Ronaldo's winner. The quality of his assists has matched the quantity this season (six in five Champions League games) but his efficacy has waned in the Premier League, where the speed of the game is quicker.
Jadon Sancho, buoyed by two games in two games and with four years experience of German football, should welcome Rangnick's arrival.
The best coaches of the best teams in the Premier League have developed a full-back into a playmaker. Reece James, Trent Alexander-Arnold and Joao Cancelo are defenders by trade who would enhance just about any attack through their technique and timing.
Making a playmaker out of Aaron Wan-Bissaka would be beyond even Pep Guardiola. At Chelsea, Michael Carrick had to instruct Wan-Bissaka where to stand and Darren Fletcher was also on his case. You could count the number of genuinely noteworthy assists the right-back has delivered on one hand since his £50million transfer to United in July 2019 and too often he hits a force field at the halfway line.
The elite full-backs are judged on their attacking rather than their defending and Solskjaer and his staff were unimpressed by Wan-Bissaka last season. It is not his fault United bragged about settling on him after scouting 804 right-backs but Wan-Bissaka's shyness is a curse.
Ethan Laird has gone under the radar from the 'Class of 2019' - when ten academy graduates debuted under Solskjaer - but is developing promisingly on loan at Swansea. Rangnick may never get to coach Laird but the bold call would be to recall him from Swansea in January and parachute him into the team.
Diogo Dalot is not regarded as a credible competitor for Wan-Bissaka by the current staff but could turn Rangnick's head over the next six months.
Player and staff futures
Rangnick technically starts his consultancy role on July 1 - the date a player's contract expires - and United should know long before then about the fate of several players.
Jesse Lingard and Paul Pogba will both be 29 come the end of the season and there seems little point in deliberating their futures, even with the appointment of a new manager. Lingard cameoed at Chelsea but does not stand out as an obvious beneficiary from the managerial change and discussions over a new contract (pointless when he was not getting a kick) are over. Pogba's intention for the best part of the last four years has been to engineer a move away.
If Lingard is serious about forcing his way back into the England World Cup squad, then he needs to put in some face time during the March and June squad get-togethers. Rangnick's recruitment track record could be key in United becoming active sellers, a skill they have failed to master for decades, and Lingard is a sellable asset in January. For Pogba, staying put until the summer would leave him with more options.
Juan Mata is open to leaving permanently in the new year and Edinson Cavani's second season syndrome already points towards a South America homecoming in the summer. Phil Jones, Lee Grant and Mike Phelan have become part of the furniture and clearing the decks would be appreciated by the next man at the wheel.