That's led to plenty of speculation as to whether or not Lesnar will actually compete at the Royal Rumble. According to Dave Meltzer of Wrestling Observer Newsletter (subscription required), Lesnar will wrestle at Royal Rumble, but it's unknown if he'll be putting the championship on the line.
Lesnar has already feuded with Samoa Joe, Braun Strowman and Dean Ambrose, and is likely going to face Roman Reigns at WrestleMania 34. With Finn Balor scrapped, it's anybody's guess who Lesnar would face at Royal Rumble.
The Beast last wrestled at Survivor Series against WWE Champion AJ Styles, whom Lesnar defeated with relative ease. It's possible he could face another superstar from SmackDown, or WWE could have another part-timer like John Cena, Triple H or Raw GM Kurt Angle face The Beast - though the latter two are probably long shots.
Lesnar is set to appear on the Dec. 18 episode of Monday Night Raw, so we could get an update on his next opponent soon. If it's Intercontinental Champion Roman Reigns, then we can expect WWE to start building up towards their match on The Grandest Stage of Them All.
With rumors of another Superstar Shake-Up, Lesnar could even move over to SmackDown and face a star like Jinder Mahal, Shinsuke Nakamura or Kevin Owens - though that is also probably unlikely.
Lesnar has been Universal Champion since beating Goldberg at WrestleMania 33, and has only lost one match over the past two years (to Goldberg at Survivor Series 2016). As one of the more dominant champions in recent memory, we can probably assume that Lesnar will once again reign supremacy in his Royal Rumble match - no matter who he faces.