That happened to me, and my first post here was the same question!
It'll be fine. I used a little duct tape to hold the airlock, but you don't even have to do that. That was about 265 batches ago, so I'm more relaxed now.
Now I always put the airlock on, and then but the lid on the bucket. I sound like "Rain Man" muttering to myself "airlock goes on the lid, then the lid goes on the bucket" over and over until I finish up. I bought another $.35 grommet (just in case!) and it never happened again!
Trust me- it's fine. It's better to leave it in there, rather than trying to fish it out! When your beer is finished, the grommet will be buried deep in the yeast cake. Just wash it, and it'll be ready to sanitize for the next time. It's harder to remove the rubber bung from a carboy- but that happens all the time too! Just push a little bit, and the wet bung will plop right through the opening and land in the carboy. So, buy a couple of extras (and an extra airlock), then you will be prepared for anything!
That reminds me. A couple of years ago, I racked and bottled a beer. When I cleaned out the fermenter, an airlock was lying in the bottom. I still have no idea how it got in there. I assume I had everything that I had to sanitize laying out, and somehow it got sanitized and in the bucket. The beer was perfect.
