You won't get many balanced replies in this thread as it contains an opinion on one of the team scapegoats, and some of our fans like to write a player/manager/coach off before they've had a chance to represent the club in whatever role.
After seeing him for half a season now, if he had been brought in as a squad player that was used when necessary then he would have received far less stick than he has. Unfortunately, Koeman brought him in with the intention of having him as our first choice right back and also as a backup left back. He was wrong on 2 counts because Kenny should have been in line ahead of him at RB, and Koeman gambled on Baines not getting injured.
At Left Back this is where Martina has been truly exposed as a weak link and part of it is down to his own lack of ability and the other part is extremely unfair on him as he is being expected to play out of position on completely the wrong side for him. You wouldn't have seen one footed full backs like Ashley Cole or Roberto Carlos being asked to play Right Back for 12-15 games and you wouldn't see Kyle Walker these days being expected to regularly play at Left Back yet Martina is being expected to do that and perform well in a disjointed side. The fact that Koeman and now Walsh twice has allowed the left back situation to continue to be neglected is a disgrace.
Martina has done well in certain games this season in doing the best he can and has contributed to some good defensive performances like against Chelsea at Home. However, I seem to remember him getting slated by many that day because some of our fans simply cannot ever go against their preconceived opinion.
That said, he is simply not good enough to be starting at this Football Club in any position. I don't think he was ever good enough, even at Right Back and is certainly not good enough at Left Back. He has regular moments of being poor defensively which include losing his man, backing out of tackles, poor positioning, failing to stop crosses, and heading the ball back in to danger. All vital parts of defending at the top level. His most glaring weakness at LB is his inability to even want to attempt to use his left foot to any sort of acceptable level. I've seen him pass the ball out of play 5-10 yards for an opposition throw in as he has a complete lack of confidence in his own ability to be able to clear the ball properly. That is not acceptable for a professional footballer expecting to hold down a starting position in the Premier League.