Matt Kilgallon was the only defender they had on the bench, he'd been an unused sub in all their previous games and had only started their Carling Cup game. He's a Centre Back who can play full back but I don't think he's that quick so that's more than likely why they decided to play Marshall there.
They had two players taken ill the night before the game, Adam Henley and Jason Lowe. Henley is a full back and Lowe is a midfielder who can fill in at full back. That is why Marshall had to play fullback. They only named 6 subs so I am guessing they only had a squad of 19 players travel down.