When the PlayStation Network came back up, many gamers were hoping to dig their claws into games that had come out when the disaster first struck. Unfortunately, Mortal Kombat needs an online pass in order to play online, a pass which had to be purchased from the PlayStation Store.
Because of this, NetherRealm Studios has waived the need for an online pass until the PlayStation Store returns to normal service. However, when it returns, gamers will need to still validate their copy of the game.
According to a memo sent out to PlayStation Network developers, the PlayStation Store is expected to come back up on the 24th of May. Still that gives owners of a used copy a few more days to check out the game's online component.
Source: Official Facebook Page