Beterbiev is no stranger to injury. Many of his fights have been postponed due to injury. This is why he has so few fights for a 39 year old.
I've just read the news. What a bummer.
But yeah, the Wilder Vs Zhang is a good fight on its own to be elevated to the main event. To be fair, it seemed a bit odd that it was on the undercard.
I read that the injury will put Artur off till at least September, which is a long time. He needs time for the knee to heal properly, rehabilitation, and another preparation camp.
This could mean we might not see the fight until very late in the year.
If the fight is indeed "postponed", as the media report (and not just cancelled), then I don't think Bivol would be allowed to fight anyone else before then. The whole purpose of it is for the winner to "take it all", so if Bivol was to fight someone else and lost, he wouldn't be putting anything on the line.