When you say top are you talking about the whole collection cup, or just the cover to the collection cup? If your just talking about the cover to the collection cup it won't really be a big deal. You might want to find something to cover it with for the time being, just cause it's going to splatter water everywhere.
If you are talking about the whole collection cup, I would not be running the skimmer with no collection cup on it. Thats just a disaster waiting to happen. :eek2:
-RYknow