We can't blame the midfield for the ridiculous numbers of goals shipped from set pieces.
To be fair, we have struggled to field a consistent back 5 (including the goalie) all season, and that does make a difference. Constant reshuffling will never improve a a defensive unit.
For me, though, there also seems to be an issue with concentration. McManus has been a good influence overall, but there are times when he (and the other defenders) seem to lose focus or switch off during a game.