@AjaxCunian and
@tomaldinho1 explained it perfectly.
Fans saw a centre back who is one of the most decorated players in football display good box defending in a struggling team, playing in one of the lowest defensive lines across the league, and completely ignored or just straight-up never realized his shortcomings and deficiencies, which led to "Rolls Royce" and "best defender in the league" shouts when he was probably never even among the top 10 PL centre halves at any time during his United spell, and both Maguire and Martínez are much better CBs than him. The best centre halves in the league are genuinely in a different stratosphere to what Varane was at United.
But we are talking about the fanbase that were led to believe that van de Beek was some world beater sitting on the bench because Ole was too stupid to know how to utilize such a genius on the pitch, and that any half-decent manager would bin Bruno and play him instead.