I don't see the reason for the comparison because these two teams are operating in two different levels because United is currently undergoing a rebuilding and is just trying to get back to their feet united has changed different managers in a space of about three years that will help you to know the current state that Manchester United is united is not fighting for any title as far as am concerned
Like you rightly said, Arsenal is operating at a very higher level than Manchester United, although suffering multiple injuries of their best men like Bukayo Sake, Haverts and G. Jesus. They've suffered some setbacks this season and even with that, Manchester United can only dream of measuring up. Manchester United is out of the race for the title completely for now until they find their feet.
Arsenal might not win the trophy for this season with the way in seeing things. Liverpool isn't getting weaker in gotten anytime soon and with the way they demolished Newcastle yesterday, they still got fire in their loins. Maybe next season they would, but this season is for Liverpool..