After a slow start to the season, Lisandro Martinez was just beginning to find his feet. The Argentine had started 14 of the last 15 games under Ruben Amorim, scoring twice, adapting to his new role impressively and bringing balance to the back three.
There's an argument that the three-man defence is the only aspect of Amorim's system that actually works. Well, at least it was working until Martinez departed Old Trafford on a stretcher during the second half of Sunday's defeat to Crystal Palace.
The wing-backs continue to look ill-at-ease, and the holding midfield conundrum is not quite solved, with Manuel Ugarte being excellent out of possession but struggling on the ball. The options at No. 10 are plentiful, but there has been no continuity, and the less said about the situation with the strikers, the better.
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At the back, however. There was improvement. There had been back-to-back clean sheets, and players were adapting to roles. Martinez's goals from open play against Liverpool and Fulham were an example of his ability to drive forward, while Harry Maguire looked comfortable and commanding in the middle of that line.
That left Leny Yoro and Matthijs de Ligt to rotate on the right. Yoro is settling in after an injury-hit start, and De Ligt has shaken off his early rustiness at Old Trafford.
But as was the case last season, so much of United's defence works because of the balance that left-footed Martinez brings. His aggression and character is a trait not visible enough in this squad, but his quality on the ball helps with build-up play as well.
A back three consisting entirely of right-footed centre-backs is harder to pull off. It can work, but it makes every passing angle a predictable one. It is even worse when the wing-backs are both right-footed as well, which makes the addition of Patrick Dorgu for an initial fee of £25m this window a crucial one.
The loss of Martinez will still be keenly felt. He had been one of United's best players of late. Yoro, De Ligt and Maguire could form the back three now, with Noussair Mazraoui potentially dropping in with Dorgu taking the left wing-back role and Diogo Dalot switching to the right.
Maybe the most obvious solution is to play Luke Shaw in that position. It is where he has featured as a substitute under Amorim and has played it many times for United.
The stumbling block is his wretched injury record. United continue to proceed cautiously with his recovery and it is two weeks short of a year since the 29-year-old last started a game for the club.
When he is fit, Shaw would be an ideal stand-in for Martinez, but the idea of him starting games regularly still feels several weeks away. That leaves Amorim having to find solutions elsewhere. The last time Martinez didn't start was against Brighton, when De Ligt, Maguire and Yoro were the centre-backs. United lost 3-1 and looked as poor in build-up play as they did defensively.
The Martinez injury is just one more headache for Amorim in a season that is giving him an almost permanent migraine. He will use the free weeks ahead of him in the calendar to try and find a solution.
