Well I was considering this earlier on. I suspect Browning and Garbutt are now considered first team players so I can't see it being them. In order of likelihood I have listed below who I think it could be.
Pennington
Ledson
Galloway
Long
McCaleny
Jones
Green
Dowell
Walsh.
I think Pennington is quite nailed on, Martinez has intimated as much recently, that if he wasn't injured he'd have been involved.
Ledson because Martinez is clearly a fan. Galloway as Martinez bought him and we will probably need centre backs at that point.
As for the rest it's a bit open for question. I don't really think McCaleny will make it and has done very little at Cardiff. Long has done ok at Brentford, but not played enough, but may be given a chance. Left field selection is Jones, he's gone out on loan and seems to have done well.
Dowell and Walsh are still a couple of years away from being in the reckoning I imagine.