Cordwainer Smith's The Game of the Rat and Dragon is a good story to get a perspective on this (and it's beautifully written too.)
People are going out in space where an in-imaginable horror awaits to hunt down and kill ships. A human mind that touches this horror goes insane in a fraction of a second. The people see/imagine/picture this horror as a great Dragon and there is no way for a human alone to shield from it.
So imho, the question is not "can I invent/picture/imagine something new" but IF I invented something new, who would understand what I just invented? If it takes human referral points for something to be understood or at least perceived as understandable, then there's no need to invent anything out of this world. All the alien movies or books show aliens similar to something we already know (insect-like, mammal-like, reptile-like etc.) If we actually had some real-life aliens (I'm not sure we should want to meet them in this timeline yet, btw), would we even recognize them as aliens?!...
The best aliens-not-understood I can remember are Cherryh's knnn - no one knows what the heck they are and what they want and what they do, and they're still a space-faring race. And they still get described in terms a human might use to form a mental picture of them.
What use a new description if it's not understood by anyone?...
But I don't agree with being unable of imagining things we've never seen. I can imagine a dragon just fine and I've never seen one. Or an unicorn. Or a hydra. Or even a black-hole.
What I am pretty sure he means to say is, we can't describe things we imagine that no one else has, before, without using things already existing. But what would be the use of describing a wherethant'ila's that fre'ns'ds all the merd'sh when you just want to say the horse eats his oats all day?!