No alternate has been named for the title bout at this time by the UFC, but former WSOF Lightweight Champion Justin Gaethje is willing to fill that role.
Gaethje said the following to Luke Thomas, which Thomas reported via Twitter:
.@Justin_Gaethje tells me he isn't against the idea of being a substitute in the event Ferguson or Nurmagomedov falls out, but it's not his preference. Would need UFC to cover cost of a training camp. Still prefers to get winner of that bout.— Luke Thomas (@lthomasnews) December 2, 2019
The duo of Nurmagomedov and Ferguson have both been booked against each other multiple times during their UFC tenures, but those bouts always fell apart due to injuries or other issues.
If Gaethje does fill in, it would lead to a potential interim lightweight title bout or lightweight title bout depending on if the champion or challenger were injured and needed to be replaced.
UFC 249 takes place on Saturday, April 18 from the Barclays Center in Brooklyn, New York with Khabib Nurmagomedov and Tony Ferguson headlining. Fightful is providing live coverage of the event, with a post-show podcast to follow.