I've put a mostly bench and reserve team in for a League match against a low-Quality bottom team today and got matches against a lower-Quality team for the Associations matches tomorrow - he'll get a surprise when he faces a team more than 15% better than he thought on match day.
I've been matched against what was pretty poor opposition on Friday that turned out to be 10%+ better than me on Saturday (bl**dy Tankers) several times - and still got 4/6 points from both matches.
It appears that only the Association matchup is done on the average Quality of the best players in teams over the week, the individual matches still seem to be on the Starting XI when the draw is made.