Forza Motorsport 5 will require an online connection at least once to play, as you'll have to download the rest of the game to compliment what is on the disc. Speaking to IGN, Turn 10 Studios head Dan Greenawalt explained how the process of getting the game running will work.
“So when you first boot up the game, we’re going to ask you to log in,” he said. “And when you log in you’re going to get the Drivatars and you’re also going to get a whole bunch of content: tracks and cars. Our production schedule is such that we are putting them in as late as possible and that means making them free as downloadable content on Day One. [But] that is required content to play the game.”
Basically, in order to have Forza 5 available at launch, Turn 10 will be pressing incomplete discs in such a way that it will have the extra month or so to keep working on the game, allowing users to immediately download the rest of the game at launch. Greenawalt likened it to a refrigerator, in that you'll have to fill it once to make it complete, at which point further updates become optional because the initial food is infinite and perpetually unspoiled. I'd like such a fridge. It's a rather clever workaround, especially because Turn 10 likely thought the always-online requirement would make the required day one DLC a non-issue.
Now that the Xbox One can be played offline (aside from a one-time required connection), this does raise a couple concerns, though. For example, what happens when servers are inevitably shut down and Forza 5 discs are rendered unplayable because the required updates are unavailable? There is also something mildly uncomfortable about buying a boxed videogame that is actually incomplete and non-working until you update it. Such is the result of trying to fortify that launch lineup.