Varane was available for at least 400 additional minutes also both Maguire and Lindelof are adequate backups. The only actual issue exposed here is the left back position.
I did some analysis in another thread which compared 'no. of starts in each position' for defenders at United, Newcastle and Aston Villa.
Aston Villa
Pau Torres started 27 / 38 games
Konsa started 35 / 38 games (some at RB I know)
Diego Carlos started 20 / 38 games
Lenglet started 14 / 38 games
Mings started once and Chambers started once.
At LB, Lucas Digne started 27 / 38 and Moreno started 11 / 38.
Conclusion - Pau Torres, Diego Carlos and Clement Lenglet basically shared CB duties with Konsa filling in the odd game. All natural CBs, all experienced professionals in their prime.
LB was either Lucas Digne or Alex Moreno, both natural LBs and senior professionals in their prime.
Newcastle
Fabien Schar 36 starts from 38 games
Sven Botman 17 starts from 38 games
Lascelles 16 starts from 38 games
Paul Dummet 5 starts from 38 games
Dan Burn started 33 from 38, presumably mostly at LB.
Keiran Trippier started 28 from 38 at RB
LB and RB were basically covered by first-choices Burn and Trippier, with Livramento and Hall (both £30m purchases) acting as natural understudies.
Conclusion - Schar was virtually ever-present and was partnered almost exclusively by either Botman or Lascelles. All natural CBs, all senior professionals in their prime.
No issues at full back. Burn and Trippier available most of the time, covered by two good young natural full backs when not available.
Manchester United
Martinez started 8 / 38 games
Varane started 16 / 38 games
Maguire started 18 / 38 games
Lindelof started 14 / 38 games
Kambwala started 3 / 38 games
Johnny Evans started 15 / 38 games.
Casemiro started 5 / 38 games at CB.
Luke Shaw, our first choice LB and arguably 5th choice CB started 12 / 38 games.
Malacia, our 2nd choice LB, started 0 games...in fact, didn't play a single minute.
Conclusion - bit of a car crash! Not one of our defenders started more than 18 games. That in itself is in stark contrast with Villa and Newcastle, who both had at least one first choice CB available most of the time.
You can also see the sheer number of players used regularly at CB is in stark contrast to Villa and Newcastle. Both used three, in the main...and the period that another player had to fill-in were highlighted by the press and demonstrating an "injury crisis". We used five on the regular, plus had to have other lads fill in at CB or be prematurely promoted from the Academy.
What you can see is a total lack of consistency and stability. Kambwala wasn't ready, Casemiro should never be at CB and Evans did great, but under normal circumstances would never be starting 15 games for a top-half team...or even really any PL team at his age.
LB is a car-crash. We basically didn't have one for more than two-thirds of the season. We ended up loaning a bit-part player from Spurs, who was also injured / ill for a good chunk of his time at the club.
If you dive even further into the stats, you can see that our problems didn't end with pre-game selection, but that we were forced into changes in defence game after game.
Martinez was subbed off 6 times from those 8 starts.
Varane was subbed off 4 times from those 16 starts.
Evans was subbed off 7 times from those 15 starts.
Lindelof was subbed off 7 times from those 14 starts.
At LB, Shaw was subbed off 5 times from 12 starts and, as we know, Malacia didn't play at all.
Some of those might have been tactical but a good chunk were in-game injuries, causing further disruption in the backline.