Can we just clear up a few things? Diuretics don't mask steroids, they mask the use of steroids by making you excrete excess steroids and their metabolites.
Second, Warne lost weight after about 2001 when he started bowling really well again. It was a fairly dramatic amount of weight too on his body and partcularly his face. The only way he would be able to use any type of steroids and lose weight is if he wasn't doing any exercise at all and that's only because then the steroids would have had very little effect i.e. he'd just whizz them out. Even then, if anything, he would be more likely to gain a little. This is a key point; no-one uses steroids, does the amount of exercise Warnie would have been doing as part of an elite cricketing team and loses weight. If you take steroids and exercise regularly, you'll gain weight.
Third, the use of steroids aids repair of muscles and Warnie's injury before the 2003 WC was a mechanical ligament and tendon injury (rotator cuff from memory?). Steroids' utility as a repair agent for non-muscular areas is still pretty hotly debated and even then, they just alleviate the pain and further damage of the area, not restore prior function. The areas in questions still have to repair themselves. Warnie's quick recovery from the injury was due to his powers of recuperaton, good physio work, etc. Any doctor who prescribes a steroid and claims it'll repair an injury quick enough to compete in an international sporting event is lying.
All that said, I'm pretty sure Warnie was lying about how long he was taking diuretics for. He lost not only weight on his body but in his face, etc. I wouldn't say Warnie or any athlete would be above juicing either. But to say he was using them to mask steroid use is not only unproven but counter-intuitive.