Why?
We kept the second most clean sheets last season and had the 4th best defence in terms of goals conceded - 6 worse than the best, Spurs.
This is with a manager who used the full backs as wingers and who's defensive midfielder was Yaya Toure.
If we had the exact same defenders as last year I'd be confident that under Guardiola we'd concede fewest goals in the league.
Our problem last year was not goals conceded (which remained fairly the same over Pellegrini's reign) but goals scored where we fell from 102, to 81 to 70.