What if Austria-Hungary detained Serbian Field Marshal Putnik on declaring war in August 1914?

  Well it definitely helped Serbia in that they have a recognised and capable military leader.  Judging by the comment about Prince Alexander seeking to replace him in early 1915 you could have had the army under royal control and the leadership might have been significantly worse.  On the other hand he was already a very sick man and a younger general being prompted to the role could possibly have done better in some ways.  However not clear how much Serbian resistance could have been extended or shortened either way once the Gallipoli campaign failed, which pretty much ensured Bulgaria joining the CPs and the fall of Serbia.  

  Wasn't aware Putnik had suggested a pre-emptive strike against Bulgarian force but given the casualties the Serbian army had already suffered and that German forces were involved in the attack from Bulgaria I can't see that having much hope of success.