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
Well, that's not surprising at all and I even predicted that Liverpool would do this in yesterday's match against PSV. So I missed my bet for that match. Because I already suspected that Liverpool would not play their best squad in that match. Because a more important match awaits Liverpool this week, namely against Bournemouth. Bournemouth is no longer a team to be underestimated this season. So Liverpool ensures that their best players get enough rest time and are better prepared for the Premier League match. After that Liverpool also have a semi-final match in the League Cup against Tottenham. So Liverpool made the right decision about resting their best players in yesterday's match.