mmmmm,
depends how big this bit of rubber is.
If its a bit as in "tiny" sliver from the rubber o ring that goes around the belmouth as it clicks into the injector body, I would say you should be ok. It would burn up.
If its hard rubber. Or getting on for more than a tiny bit. I would say that it shouldnt be in there.
I would be looking for ways to try and get it out. After all, dropping things into the cylinder head is a no no, and really does go against the grain.
What ever happended to covering these sorts of holes when working on the bike? Good workshop practice.
Hope you sort it and dont spend to much money.
DC.