...But anyway, wouldn't such an infection be visually-observable long before amputation has to happen?
Didn't the guy ever look over his body for discolorations, sores, whatnot?
Having had staph infections (been a staph carrier since my early 20's, thanks to a shower in the wrong place), yeah; if you aren't paying attention to it, it'll easily get out of hand.
When it does, someone - well, someone who's mind hasn't been addled by the routine abuse of substances anyway - who had it would know within hours. The affected area swells, feels hot and turns bright red, and it becomes quite painful, like you expect the skin of the appendage to split like overripe fruit. Left untreated, the affected area will expand quickly, and eventually the reddened area will become necrotic (develop gangrene), which becomes evident by a greenish-black hue overtaking the red swollen area.
The sad thing is that when caught early, the infection is ridiculously responsive to treatment. The first time it cropped up on me, a two week course of amoxicillin knocked it back into a dormant state for over 4 years. On the second recurrence, it wasn't responsive to the first antibiotic they tried, and so a 2-week course of Keflex was necessary. There hasn't been a flare up since that time (mid-1997), though my doctors confirm that I remain a carrier, and likely always will be. All that means is that I have an added level of vigilance required of my hygiene habits in order to prevent the spread of the disease to anyone else, and make conditions as inhospitable as possible to new flare ups of the infection.