4 superstars who might replace Braun Strowman to face Lesnar at Royal Rumble 

#2 Baron Corbin

If Corbin gets defeated by the beast incarnate, it's him who will get hurt lesser (career-wise) than anyone else right now

Baron Corbin helped Brock Lesnar to win the Universal Championship at WWE Crown Jewel PPV by attacking Braun Strowman with the title from behind. Although it was clear that Lesnar didn't need his help, Corbin may challenge Lesnar saying that it was him who helped Lesnar to win the Universal Championship again.

And as he's the general manager of Raw, he might use his authority to put himself straight into the Universal title match at Royal Rumble without taking participating in a qualifying match.

So, if Strowman will be out of action for a long time then making Lesnar defend his title against Corbin at Royal Rumble PPV will be the great move as if he gets defeated by the beast incarnate, it's him who will get hurt lesser (career-wise) than anyone else right now.