Edvinsson was not ready day one, and he has admitted as such. This past summer he was coming off of surgery and needed the time in the AHL to get rid of some bad defensive habits. In particular he was too reliant on his reach to defend and not learning to defend properly. He learned well and looks like he will be very good for it. His interview with Daniella Bruce confirms this.
I understand that Petry and Holl were horrible to watch all season, and that Detroit fans would rather have watched Edvinsson be there all year instead of Holl. But Yzerman had a clear plan of succession in mind when he brought veterans in, to help guard against the younger players not being NHL ready. Holl's failure, combined with Walman's injury allowed for Edvinsson to get a chance and he made the most of it.
Other young players will have a similar situation to succeed, in the future, but Yzerman will make sure that his team is still competitive and that the young players have the chance to develop properly. That is the sign of a competent GM. That he was able to get 91 points, with a lesser skilled team, and continue to build the team properly is what every franchise aspires to. Just ask Buffalo and Ottawa.