Lindelof starts in PL this season: 2-1, 1-3, 2-1, 3-0, 3-0, 2-0, 2-2, 1-0, 0-1, 0-1, 2-0, 1-0, 4-1
Goals conceded when Lindelof came on as a sub 1, 1, 3,
Total 15 goals conceded in equivalent of about 14 full games
Discounting games against Man City away, Liverpool away, Newcastle away and Arsenal away, 11 goals conceded in equivalent of 13 + a bit full games
Martinez started those heavy defeats. If you discount those games, 18 goals conceded in his 20 other PL starts
It looks to me as though United's defensive problems in the last few years have far more to do with the work-rate and defensive competence of our attacking players than the names of the CBs. To give an example of this, Hall created two huge chances simply by advancing up the pitch, receiving the ball then crossing to Mudryk and Havertz, because first Antony, then Sancho, failed to track his run from deep.