I think that this time we will not see such a surprise and the fact that Liverpool lost against Atalanta is because they did not took the adversary that seriously and when they wanted to change things in the return leg they simply could not. Now the team of Liverpool is much different compared to last season and I think they will go far in Champions League and maybe even in final as this team is showing awesome performance so far.
If it happens that they come in Europa League I am sure they are going to win it this time.
When Liverpool qualified for the European League, they did not consider the opponents of this league very important because they felt that Liverpool was much stronger than the teams that qualified for the Europa League and they could easily beat them. Due to their over confidence, they were eliminated from the quarter finals. However, the team that was not considered in the list of the best teams in the European League won the European League. Hopefully this season Manchester United won't feel too strong and underestimate other teams. Manchester United will surely consider each team as a tough opponent and will plan one game after another but they can end the tournament with a good result in the Europa League.
You are right, Liverpool was in good form last season and they were having a very good performance in the Europa League competition as they were seen as one of the favorites to lift the trophy but they underestimated their opponent Atalanta thinking that they can easily be defeated but they eliminated Liverpool because they were able to score multiple goals in the first leg of the match.
Coming to the performance of Manchester United this season, their performance alone is worrisome it will make them to see any team as a very strong opponent since the team is not in the best shape since last season, they are going to try to show their best performance in every match they play in the Europa League competition with the hope or getting to the finals.