For a long time, power plays have been evaluated using PP%, calculated by dividing the number of power play goals a team scores by the number of power plays it has.
Using this metric, Detroit was 24th in the NHL last year, converting on 17.5% of its power plays. The Red Wings were even lower the previous year (27th out of 30). Seeing that the team was 13th and 2nd the previous two years respectively, the last two years have been a giant step back for what used to be a team strength.
In his article What the Red Wings can learn from an anatomy of a successful power play, Prashanth Iyer points out that the loss of assistant coach Jim Hiller corresponds to the decreased effectiveness of Detroit’s performance with the man advantage.