Are you claiming that Manchester United were able to finish in a decent position just because they played against weak teams? I can't consider any team weak in this tournament because they qualified for the Europa League. Which weak team does Manchester United face during the group stage? Can you say Rangers, Porto and POAK aren't strong teams? Perhaps you mean Bodoe, Twente, and PIzen are poor teams. Did you believe that if Tottenham Spurs had played against all of the teams that Manchester United defeated in the group stage, they would have finished fourth?
However, I do not think you watched Manchester United's game against FCSB; even if you did not, I believe you should review the game numbers to determine which team dominated during the game. Manchester United controlled FCSB throughout the game, but they were not able to score a goal in the first half. Let us wait for the next round of the game to see how Manchester United plays, but at this point, I am confident that they are going to make it through the next round if they continue in this way.
While I do agree that there were better teams to face, we have to accept that any team here is a big team, ok maybe the last 6-10 range isn't that great, so if you faced any team that is at the bottom 10, then yeah they might not be good, but the rest are definitely all good teams without a doubt.
United only manage to finish at a good position, not because of where they landed, but because we are seeing them being a tournament team yet again. They did that with EFL a year ago, they did that with FA cup last year, and they are doing it with Europa this year. While they are terrible at premier league, and have been for a while, they are pretty good when it comes to tournament structure, no idea why there is a big difference, but it has been true for a while.