Long story short we lost due for 2 reasons:
1: Our Lack of Strength in Depth - Lets face it a big reason why we lost that game was the fact that by their own standards thoughout the season, they simply underperformed in this game. Something which I highly suspect is due to the fact is due to the number of games the same players (more or less, especially the likes of KDB) have had to play thoughout the season (due to the lack of strength in depth) while trying to maintain the same level of high performance at the same time. Which at the end of the day to take its toll on the players in question and thus result in them underperforming as we get deeper into the season, hence the defeat to Liverpool.
The only solution for is is to make sure that the players we have in the Second XI are as good as they are in the First XI (through transfers or even from the academy), that way you reduce the number of games the First XI (or at least some of its players) have to play thoughout a season and thus give them much more time to recover, which in turn allows them to maintain or even improve the level of performance they put into each game. I mean if you had the likes of say Lloris, Pogba, Alli, Hazard, Kane and Sanchez play against Watford (in the League) and Bristol City (in the League Cup) while having Ederson, D.Silva, KDB, Sane, Aguero and Sterling play against Burnley (in the FA Cup) and Liverpool (in the League), I would be very surprised if we where unable to convicingly defeat both Bristol City and Liverpool under those circumstances. Simply because in both those games the players would have more energy to put a high level of performance in those games.
2: Liverpool discovering the only way you can beat us...and somehow doing it without Coutinho - Considering the attacking and midfield players (which is without a doubt the strongest in any First XI, if only we had more of them though...) we got in our team and how they performed this season, parking the bus is simply not going to work and the PL teams that have tried it have without exception failed to beat us. Rather (as Monaco showed against us last season) the only way you can beat a team like us is to ourscore us and despite losing Coutinho (and thus an important part of their attacking potential) they somehow managed to over come both his loss and the negative impact on the teams morale that it brings, so credit to them for overcoming that and capitalising both on our still weak defensive and the lack of strength in depth that we have.
However that does not mean that they are on course to win the title in the next few seasons or have secured a place in the Top 4, simply because even with VVD in the squad, their defensive is still nowhere near good enough (hence why they almost blew the win at the closing minutes of the game, if only we had Sanchez on the bench or the field, then at least we would have got a draw out of it if nothing else) and their midfield needs room for improvement as well. To which they now have the problem of no decent backups in their attack either (which cost them badly last season with the Mane injury).