An email purportedly received from Warner Bros. has revealed the publisher may jump on the online pass bandwagon - but isn't advertising the fact.
Joystiq got its hands on an email which appears to be a communication from the publisher to retailers.
The email details an online pass code included as an insert with new copies of the game, which will cost around $10 if purchased separately.
Players with a second-hand copy and therefore without a code will apparently get a free two day trial on online multiplayer before being asked to pony up 800 MS Points or $9.99 on Xbox Live or the PlayStation Network respectively.
Joystiq reports the email goes on to explain that the online pass won't be featured on the game's packaging or advertising materials, and that both F.E.A.R. 3 and Batman: Arkham City will sport a similar pass system.
In the single-player only Arkham City's case, the pass is thought to give access to premium content of some description.
We've asked Warner Bros. for comment. EA and THQ each employ online pass systems for major releases.
Mortal Kombat releases in mid April for PlayStation 3 and Xbox 360.