Predicting that he's not going anywhere is probably the safest prediction in football. So what if he isn't, does that mean he gets off without any critical observation of his under performance?
Ultimately our youth and inexperience (if you wish to see it this way) is Martinez's choice. Coleman has neither issue so there is no excuse for his rank form, Stones has had two and a half seasons now and is a full international, when does the 'inexperience' line wear thin? If he's not learning why is he in the first team? If the defence was crying out for experience why did Martinez go and buy a player with no prem experience in Mori? I don't think he's a bad player by the way. Equally why did he select Galloway over Baines or over Garbutt who is on loan.
My main point is people need to stop divorcing negative outcomes from the manager's decisions. If we have a young team, its because Martinez has tailored his transfer model that way, if we have a poor keeper its because Martinez keeps selecting him, Howard's 'individual errors' are Martinez's every time he keeps picking him. If we have an inexperienced defence that is the outcome of Martinez's selections and transfer policies. Otherwise the argument can be thrown the other way to say that Lukaku would score whoever he played for so Martinez doesn't deserve credit for that. That's illogical, the same way disassociating all the negative aspects of our team from Martinez is also illogical. He singularly owns al of the good and the bad.