I view Martin as an old school LB, he's tidy on the ball and has a good cross in him, but he's just not agile and fast enough to run the flanks on his own, and he knows it, going by his actions. He's careful where he picks and chooses his runs so he doesn't leave the left side exposed at the back.
It's a problem when you desperately need width and need him pushed forward a lot, and one mistake he gets burned. From what I've seen at CB you can see his intelligence and looks very comfortable in and out of possession.
I need to see more of him as an LCB to get a better reading as Bilbao were complete shit going forward but he seems very comfortable at least though.