Dan Greenawalt, the studio head at Forza creator Turn 10 Studios, told IGN that "when you first boot up the game we're going to ask you to log in, and when you log in you're going to get a whole bunch of content: tracks and cars. That is required content to play the game."
[Update: Greenawalt has clarified that Forza 5 will be playable on day one without the download]
Effectively, the mandatory day-one patch means that Forza cannot be played unless the game connects to the internet. Greenawalt's explanation was that content has been added late into the production schedule, though he didn't elaborate further.
It is possible that this predicament is a by-product of Microsoft's Xbox One policy reversal. On June 19 Microsoft announced that, "as a result of feedback from the Xbox community", it had decided to scrap the system's always-online requirement.
Greenawalt did not discuss the policy reversal and how it affected his studio.
Announced at the Xbox reveal event in May, Forza 5 features a flagship AI system - called 'Drivatars' - that apparently mimics the traits of real-world players.
Greenawalt said: "You do have to connect the game in order to get the latest Drivatars, because we need as many people training them as possible. And so rather than having just a launch-day set that was created by us, every day that people race is going to make the Drivatar set that much more accurate, that much more diverse, that much more interesting.
"All of the cloud and online features make the game far, far better," he claimed.