there is no way that you can blame a poor run of result on DP's form. In what way can the defensive problems be blamed on DP. He has been a loyal servant to the club and has always given his best and has been a big part in alot of juves scudetto wins.
As for Montero your right he is past his prime, he is not what he used to be. we need a more youthful defense to cope with the demands of the best league in the world and someone who is going to be consistant in the 3 competitions that juve play in.
We can still win the scudetto this season with the team we have currently. But a Defender is a must in the transfer window.