WMG / Mortal Kombat: Defenders of the Realm

Johnny Cage was one of the partners that Sonya lost.

As mentioned by Nightwolf in the first episode, something along the lines of "Sonya lost her last two partners in battle with Outworld invaders". The first would've been Wexler, her old partner, who was killed by Kano. Johnny was the other one, thus explaining his absence in the show.