There is no way to truly escape this, but there is a way to get out of this bad situation by taking minimal damage. Simply down parry the reflector on wake up. If you do this, the knee drop will cross you up and hit you high while you are parrying low. Since both hits happen at the same time, you can only parry one of them, so you get hit by the knee drop. However, since you parried the reflector, that hit is nullified and the knee drop hits you cleanly. The knee drop causes a knock down so you escape being hit by the reflector.
After being knocked down, don’t quick recover. Stay down, and when you get up, even if Urien hits you high or low, the reflector does not stay out long enough for him to continue and launch you for a new juggle. This is not a perfect escape, but you manage to get through it with only taking damage from the knee drop, as opposed to the knee drop -> reflector -> down fierce -> juggle combo -> unblockable #2 and so on.
Unfortunately, this only works in the corner. Mid-screen, Urien doesn’t need to do a knee drop to start an unblockable, so you have to guess parry your way out. Urien generally only does stand forward, or low short, so the timing is generally 3 parries and then throw out of the reflector. But you have to guess whether he goes high or low. If you escape the mid-screen set up a few times, Urien might just throw you after the first parry and juggle you off the reflector, but that’s usually better than him getting a clean down fierce.
Mid-screen reflector unblockables are pretty terrible, but they usually don’t happen unless you did something stupid. Corner setups on the other hand can happen whenever Urien has 2 meters, so it is good to know how to escape it.