It depends on Sturm really, Murray's a decent operator and strong but lack of bridging the gap in terms of fights at this or around this level maybe his undoing. In saying that he's got nothing to lose and Macklin's shown how to combat Sturm but Sturm's a accomplished operator but i wouldn't write Murray off. He's got the sheer strength and aggression to cause the champion problems. I need to see more of Murray's recent fights to gauge how good a chance he has but at this time i'd give him a slight chance.
This fight is more then winnable for murray, he is a lot stronger then macklin and frankly if sturm performs like he did against macklin he will get stopped!
You can still get 3/1 on a Sturm stoppage at Skybet. I haven't checked any other sites yet but that seems great value considering a) Murray's leap up in class, and b) the likelihood of a UK stoppage in Sturm's favour.
Sturm is beatable so I'm rooting for Murray. Sturm is fast and explosive but fights in spurts and trusts too much in his full guard. Murray has to blind him with the jab and bomb his guard in combos, step off and repeat. Also be sure to fight each round smart and not let Sturm steal the round i.e. start and finish strong.