These teams are not having to play every week in a league as competitive as the Premier League. One reason for English teams not doing as well in recent years in the Champion's League is that the PL has become more competitive and the top teams are not able to rest their whole team for games like they were doing a decade ago. Man U or Arsenal used to go to QPR or Fulham or West Brom (home and away) with their second string playing. These days they cannot do this. I think we would qualify even if we finished fourth unless we get a really unlucky draw (like Juventus having to qualify) however I think we would be looking at getting out of the group as an achievement. Man City can't even do that.
