Don't know about Drogba, but Lampard and Gerrard had those deals done months in advance so that they could come straight in without losing too much fitness. The main difference is that all 3 are world famous players on a completely different level to Zamora.
I don't think he'll be going to an MLS side now. Their season started 4 months ago; meanwhile he's not been training with any team and losing match fitness.