Totally disagree with your Mertesacker views though I can't be arsed to (wholly) go into why.....but Mertesacker's poor execution of any of his defensive duties is not him shunning his responsibility at all. In fact I think he has regularly shown responsibility on and off the field....... and in terms of character I think we are extremely lucky to have him frankly.
The line between good and bad is sometimes a thin one and Jack has exemplified that in a footballing sense this season and I actually believe he has been far more good than bad, but some of his other antics have overshadowed it and that has sometimes been his own fault.
He is one of the most talented English midfielders we've produced in recent times and one that has spent around half of his professional adult career on a treatment table......but that can be wilfully dismissed depending on how much he annoys you. In the first few games I stated he played well and had outplayed Ramsey (in spite of Ramsey scoring), but that was somewhat overlooked but I think that tone has been set as Ramsey hasn't played really well all season and Jack has been much better than many are willing to give him his due for.