From a cursory search of google, a bruised knee will not get fluid in it on its own, unless he hit his knee hard enough for fluid (blood) to leak into the bursa. If you've managed to pull that off your probably going to get an X-ray at the minimum, because you have to hit your knee pretty freaking hard for that to happen. Treatment for this is exactly the same as for a bruised knee, it just takes a lot longer.
There are other underlying issues that can cause fluid to accumulate in the knee which forms a bruise, none of those conditions are conducive to ever playing professional soccer again at age 38.
My bet is either a way for Pirlo to gracefully retire or he just hit his knee really really hard. Of the two I'd probably imagine that he hit his knee really really hard, if only because the marketing team is pretty on point and would have started to phase him out of the promo materiel if it was season ending.
