It entirely depends on the context of the professional's job. A bassist playing in a full-time symphony orchestra should be a competent sight-reader, but it's actually pretty rare for the music to be completely unavailable before the first rehearsal. Some orchestras may even have it in their contracts that the music must be ready X days in advance.
Truth be told, however, most symphony orchestra players generally aren't spending huge amounts of time on their parts. They will learn the licks they don't already have, but they won't exactly be agonizing over every little bar. In a sense, this amounts to sight-reading on the job. If you are a competent sight-reader, it will significantly reduce the amount of time you need to prepare a part.
Someone recording for movie studios probably has the highest expectations for sight reading. These parts are generally being recorded as they're made. Since the studios lose money for every wasted minute of recording time, they really will try to get it right on the first take.
As a professional freelancer, it is not uncommon to come to a gig with little or no rehearsal and be expected to sight read relatively standard repertoire. It's rare for something difficult and completely unknown to come up in these cases, but it happens. If you manage to ruin the performance, it's possible the contractor won't be calling you again.
New music ensembles will also have to do a fair amount of reading, usually when workshopping with composers. Given the enormous difficulty of many of these parts, I'm not sure what the expectations can realistically be in some cases. However, many of my non-bass player friends in new music ensembles are ludicrously gifted sight readers.
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