Martin is fast in terms of top speed, he is simply a stiff player who lacks lateral quickness.
He can return fast if needed, but ask him to change directions and he will fall on his face.
He is just a smart player and fairly cautious while being a hardworking one, you add the facts that he is a big guy with one of the team's highest top speed and you you can see why Flick has preference for him.
However, in a better financial situation you should absolutely go for someone who actually has some talent and can carry the ball and make a pass in tight space.
If he keeps the fight in him, he will have avery decent career, that likely no one expected him to have.