That's not really being honest when evaluating.
Partey would easily get into our xi, as would Martinelli. Sure their not better than Casemiro or Rashford, but wwe would definitely accommodate them. Xhaka is also very underrated. I don't think Eriksen is much better than him. White is better than Wan Bissaka.
The real flaw in your assessment is the fact that all those players mentioned are still good footballers. They may not be the best out of these two teams, but they would easily get into most teams. All of them can pass, hold on to the ball and dribble. They can do the basics of their positions really well.
At United, we have some footballers who would not start for most Premier League teams. We have some who can't beat a man in attack. Some like Mctominay and Fred who have terrible positioning and make passing look difficult. Eriksen lacks the engine or strength of Xhaka or Partey. The players they have in reserve are also decent footballers, so as a result, even with an injury, they can fill the gap knowing the drop off would be limited. At United, if Bruno goes down, we had VDB replacing him. VDB couldn't start for Everton last season. Mctominay can't start for Scotland. AWB did not even play for us for the first half of the season. Weighourst had issues at Burnley. Even the likes of Martial are pretty overrated.
I also think our fans truly underestimate the quality of this Arsenal team. Odegaard might be the best player in the league this season. Saka might be just as good as Rashford. Martinelli is a fantastic footballer and is easily better than Antony. Gabriel Jesus was better than anything we had in the centre forward position this season. Partey is likely a top 8 central midfielder in the world ( top 3 DM), whilst Xhaka is better than Fred and Mctominay combined.