Martinez was taken off for a reason that game, they were targeting him, and whilst he dealt with most things comfortably, it was a required sub to bring some respite from the bombardment of long balls.
Watch the game and you'll see that time and time again Brentford were hitting long balls to the left hand side of defence to target him, and eventually it paid off.
I think he's done brilliantly bar that game whereby he was relatively new to the team still and had probably never really had to deal with a game plan like that too often and so was slightly hesitant in places.