That attacking focus would come at a cost though for United as Reguilon is a far weaker one-on-one defender than Shaw, whose tackling ability — which you can think of like a duel win-rate that’s weighted depending on how good the players you’re tackling are — is rated by Smarterscout as 73 to Reguilon’s 13. The Spaniard would improve the side but his possible addition would ask a lot from the person playing at left centre-back (more on that later).
Liverpool and, to a lesser degree, Manchester City (more so when Vincent Kompany was there) work well with advanced full-backs because they possess mobile defensive midfielders to stop the flow of attacks when their teams lose possession. Or, failing that, a robust centre-back who can dart out of the defensive line to deal with nascent counter-attacks before they become dangerous three-v-one situations.
Manchester United’s defensive midfielders are either not mobile enough (Matic) or strong enough (Fred) or do not possess the requisite ability when in possession (McTominay) to do the tactical fouling job as well as the Liverpool/Manchester City duopoly, and neither Maguire nor Victor Lindelof possesses the match-reading or ambition to dominate high-risk one-v-ones with any consistency.
(Carl Anka and Tom Worville, "Manchester United’s squad depth: the good, the bad and the weird," The Athletic, Sept. 3, 2020)