If you buy Call of Duty: Infinite Warfare to play Call of Duty: Modern Warfare Remastered as a physical copy, you’ll need to have the disc in the drive in order to play the remastered game.
If you buy it digitally, you should have no problem running it without having any disc in your drive.
The Modern Warfare Remastered website has the following note for anyone intending to play the game. It reads:
Modern Warfare Remastered is a full game download (game disc must be inserted to play Modern Warfare Remastered).
Call of Duty: Infinite Warfare is available as a standalone title, but if you want to play the remastered Modern Warfare, you’ll need to shell out for the Legacy, Legacy Pro or Digital Deluxe edition of the game.
This latest move is presumably to prevent anyone who wants to play Modern Warfare Remastered but not the new game from trading in their copy of the game as soon as they install it. I can’t think of any other way they’d go about preventing that, so any outcry against Activision for making the move seems unreasonable.
