Our most resolute performances have come with Jags at the back. You have some mad obsession with players under 25. If Martinez gets his way and replaces all of Moyes back 4 we will likely flirting with relegation. If Mori cuts out basic errors, if Stones cuts out basic errors. Do you honestly think he can coach a decent defence? I will conceed that Jagielka's distribution is poor, and doesn't fit into 'the system' but I have no faith in Martinez’s system.
Edit: v Arsenal we were caught cold twice due to Mori being out of position. Jags the Moyes fanboy elderly coward captain didn't shout at him *tongue in cheek*
I have the opposite view...
To be successful with this style every player needs to be able to keep the ball and pass it to a team mate. The only players who are weak at passing are Jagielka, Coleman and McCarthy (Baines notably has gone downhill head also).
However McCarthys job is to pass it simply to keep the ball...hes no busquets but i doubt he gives the ball away and can pass to Barry or Stones/Mori to rebuild attacks.
If we keep the ball well and are able to play it from the back then we wont be giving the ball away so much and can plah a higher defensive line....NONE of the defenders other than Stones and Mori (possibly also Oviedo and Galloway) can keep the ball at a level which is needed = sitting too deep in defence.
As per the centre backs especially, Mori and Stones are "on paper" the best partnership. However Mori is adapting in his first season and Stones has gone through his first bad patch as a 21 year old....Jagielka is a good defender but doesnt fit the style of play. In your EDIT you mentioned Jags saving Mori....i also saw Mori save Jags 2ce against arsenal.
What Martinez needs to do in the summer is bring in HIS OWN players....he hasnt done too badly with his signings for the first eleven so far.
I think we need defenders who can defend but also who can pass the ball from the back....if we had a Marcel Desailly type (Kalidou Koulibaly'Napoli') then he would tick the boxes for me.....as would a pair of full backs who defend first and dont get caught out of position so much.
These crosses coming in should be stopped at source and were cut down when oviedo was in for baines....i wonder if coleman had competition from a more defensive right back if these crosses wouldnt also stop from the right.
Arsenal and Barca dont have pure defensive players who cant distribute effectively (Jagielka) nor do they have full backs caught out of position so much as they have been playing so long as attacking full backs which we dont really need anymore.