Liverpool never took this game serious since they have already qualify that's even why there betting odd was even big secondly they didn't play the game with some of there big boys if they played with their key players it would have been an easy win for Liverpool this is why PSV had more ball possession and more goal attempt in that game, Liverpool never saw that game as a serious one if it was a game that requires Liverpool to win and qualify loss and go home you would have seen how serious Liverpool would have been in that game I believe they would have play with all key players, let me just take Manchester City game and club brugge last night since Manchester City needed a win to qualify they put in all their best and also played with all important players to make sure they have all 3 points in that game and they won 3-1 so that's it, so In a normal circumstances it will really be a difficult one for PSV to win Liverpool in the UEFA Champions league game if not that Liverpool has already qualify.
A team resting their key players in league matches they appear on top, is very common to see in matches. Liverpool has an upcoming match to play against Bournemouth in PL. They prefer to win that match rather than playing their key players over the match they seem to have qualified in, round of 16(Champions League).
As Liverpool currently tops the Champions League table, It's gonna be a long time before they have another Champions League match to play. For the time being, their focus will be on mostly Premier League matches because they have a lot of PL matches in the pipeline