@Baldulf:
Placebos don't work with a mind like Luffy's.
If he was able to get up and keep fighting he would have done it without asking for help.
Actually, placebos are more likely to work on him than anyone else. It's been well established in the series that Luffy's simple-mindedness makes him particularly vulnerable to sneaky tricks or suggestion techniques (two key examples being Jango's hypnosis and Goldenweek's color trap).
There are plenty of documented cases where people who are under some sort of placebo effect will do better in some given activity than they did without the placebo effect (despite giving 100% without the placebo). In short, the power of suggestion brings out some potential in people that may not have been accessible otherwise.
Those most likely to resist placebo-related effects are those who have reason to be skeptical of the intended effect of what they're receiving. Obviously Luffy doesn't fall into camp for a multitude of reasons.
Whether or not Ivankov tricked Luffy here remains to be seen - but it's actually a pretty good theory. Given the fearsome reputation that Dragon seems to have in the OP world, it seems like a rather large risk on Ivankov's part to do something that (from his point of view - not knowing about Luffy's plot armor) would almost assuredly kill Dragon's son.