Kiwi heavyweight Joseph Parker is ready to step in to fight world champion Anthony Joshua, if required.
Joshua's scheduled bout against Jarrell Miller is in doubt, after the American returned an "adverse finding" in a drug-testing sample taken last month.
The fight for Joshua's WBA, IBF, WBO and IBO belts is scheduled for New Yorik's Madison Square Garden on June 2 (NZT).
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ESPN reported that Miller had tested positive for the banned substance GW1516, which boosts endurance and helps athletes burn fat.
Responding to a fan on Twitter, Parker insisted he was "very willing and ready" for a rematch with Joshua, who beat him on points to take his WBO belt in Cardiff last year.
Parker has been linked with a fight against England's Dereck Chisora in July, but a rematch with Joshua would almost certainly take precedence for the Kiwi fighter in a bid to regain his world title.
Joshua's promoter, Eddie Hearn, told IFL TV that they would have to wait for Miller's 'B' sample to come back, before any further decisions are made.
Should Joshua need a new opponent, Hearn doesn’t think Parker is a viable option.
"There are probably eight acceptable guys," he said. "Parker said he'd come on again, but I don't think that's an option.
"We got lots of phone calls from lots of heavyweights, who can smell a pound note and want to fight for the heavyweight title.
"We have to go through the genuine ones - the ones which we think are credible - and go from there."
Hearn added that there had been no discussions about moving the June date, so whoever steps in would have roughly six weeks to prepare.
He added that more than 17,000 tickets had been sold, with about 10,000 fans from the UK expected to fly across the Atlantic Ocean for the bout.
Parker met with family and management team on Wednesday night to discuss his future in the ring.
His promoter has returned from overseas with offers to secure his long-term future or turn them down to remain an independent fighter for his next bout.
The Kiwi had three options on the table, which included signing a multi-fight deal with American promoter Bob Arum and ESPN, signing a multi-fight deal with Hearn and online streaming service DAZN or remaining independent.
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