In response to notin's comment:

OK, start naming names.  20 successful (defintiion pending) big league managers who stepped in with no managerial experience and became great managers.

 I'll give you Ventura. but not Matheny, since he did inherit a World Series Champion, so his contribution is questioned

Wow Ventura gets your approval , despite his team not making the postseason while Matheny led a Cardinal team into the postseason even after losing one of the best players in baseball (Pujols) and one of his top starters out for most of the year (Carpenter) and anyone one coming back from a major injury (Wainwright).

If you ask me, Matheny did a better job with the cards (no pun intended) he was dealt versus Ventura, although for a 1st year manager Ventura did ok too.