Mannix reported Wilder's focus is starting to shift toward another opponent due to "rapidly fading hopes" of a third encounter with Tyson Fury.
Fury handed Wilder his first professional defeat last February, with the referee calling for the bell in the seventh round.
The two fought to a split decision when they first crossed paths in December 2018.
Plans were underway for Fury/Wilder III until the COVID-19 pandemic significantly disrupted the sports calendar. Top Rank chairman Bob Arum confirmed to ESPN's Dan Rafael a July 18 bout was getting delayed until this past fall.
Negotiations never gained traction from that point forward. Arum told ESPN's Steve Kim in September a Dec. 19 date was off the table because of overlap with NFL and college football games.
The Athletic's Mike Coppinger reported on Nov. 10 that Wilder was triggering a "dispute resolution provision" in the contract he signed with Fury for their rematch. His representatives were hopeful a mediator would rule Fury was obligated to take part in a third fight.
Shelly Finkel, Wilder's co-manager, provided an update on that situation to Mannix:
"In a telephone interview, [Finkel] acknowledged that a mediator informed Wilder's team that Fury was refusing to face him. The plan now, according to Finkel, is take the case to arbitration. Finkel said the plan is to file for arbitration sometime in the next two weeks.
"'We're going to go for everything we can,' says Finkel. 'I'm a realist with what I am up against. We have an agreement. Even if we didn't have an agreement, we gave Fury two shots. He has made more money than he has ever seen. Deontay was very kind to him while he was going through his mental problems. Forget everything else, morally he should give him his shot.'"
Having shifted gears from a proposed fight with Agit Kabayel, Fury is turning his focus toward Anthony Joshua. Joshua's promoter, Eddie Hearn, said last week the "financial elements of the deal are done" and that a contract should be finalized soon.
That Wilder is now lining up a matchup with Martin seems to be an admission of defeat for now.