Cinelinx spoke to someone in the know and found out that:
The basic story for the standalone was already in place, being hammered out by a few people and run through the new Lucasfilm story-group (so that everything fits together nice and tight), and Whitta was brought on to create the script based on those ideas and generally flesh it out. That was his job, and he’s been hard at work on it for the last year, which likely means this isn’t the first draft he’s done. From there, he was set to move on (which apparently he’s been off for a few weeks now) and then have someone else come into finalize the script, polish it up, and generally make sure it’s the best it can be.
This fits in well with previous rumors, that Whitta was brought on to form Kasdan’s Boba Fett/Bounty Hunter spin-off into a screenplay. The site also confirms the rumor that Simon Kinberg is writing the final script in an attempt to tone down the size of Whitta’s story to something that is actually film-able.