Maybe one of the reasons for Liverpool's defeat is that they wanted to prepare for the Champions League match. The Champions League is more important and Nottingham was an opponent they did not see as strong. They must have thought that they could beat them in any way and underestimated them. But you shouldn't do that. You should take every opponent seriously, both in the league and in the Champions League. This defeat could even cost Liverpool the championship at the end of the year.
That is not an acceptable excuse for their lost during the weekend because they had their key players in that game and wanted to win but just that they had a hard luck and Nottingham forest were stubborn enough to defend their early goal to the end. The Champions League is mostly prioritize by managers when it has gotten to the round of 16 and not just in an opening group stage match that they have to risk their domestic league games at an early stage. I think slot should be able to manage in preparing his team to withstanding both competition's as klopp has done when he was with the Reds.