Cartilage is non-replaceable, if what he says is true and he's "bone on bone" in one of his knees than we'll never see him again .

Even minor irritation down there with full thickness cartilage is very hard to deal with; without some off the bike strengthening & stretching which is counter productive to riding the bike.
Runners, football players, tennis, basketball get degradation in the knees over time probably 2-3 times faster than cyclists or swimmers..... unless the cyclist crashes and dislocates their knee or smashes the knee cap... Not a good way to go out at all for Andy Schleck. Those of us with joint pain cringe!