You're not supposed to lose your best players for 80% of the season though. How often did United lose Ronaldo or Giggs or Ferdinand to long term injuries? It's not something you're supposed to deal with.
Do you go into every season assuming you're going to lose one of your best players to a serious injury for most of it?
I think the point is just that Liverpool have a lot of quality in their starting XI relative to how effective their playing style is and that they can overcome injuries from players, but of the seven really top players - they don't really miss any time to negatively impact the team (Mane, Firmino, Salah, Robertson, TAA, Allison, Fabinho). United don't have enough quality currently or a type of way they know they can play to compensate for less quality to continue to get full results or it's extremely difficult to grind out results like it was with Mourinho's second year and it's unlikely to get those breaks and achieve those fine margins year over year.
United recently have five players who are really good, but if one goes down, especially in central midfield, they could suffer a bit more (Pogba or Bruno). If Martial goes down, then Ighalo is a good backup, but totally different player to Martial. If Greenwood or Rashford go down, James is totally different to either player. If Pogba goes down, as we saw it, the team can suffice with Bruno, Fred, and Matic, but it's obviously not the same.
United at their Fergie peak had more than enough talent in their XI and understood how they could get results with bench players in a league that didn't have the talent up and down the table it does now. Liverpool have a lot of quality players in their starting XI and they do exactly what is done for the greater good of the team, which doesn't always need the top quality players. If United lost Ronaldo or Giggs, they still had a the best defense in Europe, Rooney, Scholes, Nani, Chicha, Carrick, Berbatov, etc to get them through games and it was professional displays, seeing out leads and being in control.
If Liverpool lost TAA for the duration United lost Pogba, Liverpool wouldn't be the same but should be okay because they still have quality all over the pitch and can replace some of TAA open play output and other players pick up the slack. Losing Allison wasn't that big of a let off because having an adequate back in Adrian was okay. It wasn't Karius coming into goal for Allison.
Fabinho adds that extra class in the midfield, but a combo of Henderson, Wijlandum, and one from Lallana, Ox, Milner (very underrated), was still able to do a good job and keep functioning for the purpose of the team.