Ekstrom, a four-time individual ROC winner, was set to travel to Sydney this week to pair up with Johann Kristofferson for Team Sweden.
However that plan has been derailed by illness off the back of the Abu Dhabi Desert Challenge which has forced the factory Ford driver to withdraw from the Sydney ROC.
He will be replaced by Supercars megastar Mostert with Team Sweden being re-branded as Team All Stars.
“It is with a heavy heart that I need to withdraw from Race Of Champions 2025,” said Ekstrom.
“I was really looking forward to racing in Sydney, but last week, during the Abu Dhabi Desert Challenge, I felt unwell and had to abandon the race.
“Unfortunately, I’m still not fit enough to make the long trip to Australia and be able to compete at the highest level.
“I wish everyone a great race in Sydney and may the best champion win.”
Mostert admitted the last-minute deal was a cure for some impending FOMO.
“The Race Of Champions is such an iconic event and I was quite envious of everyone who was announced in the original line-up,” said Mostert.
“The call to replace Mattias Ekström came from left field, but it was a pretty cool call to receive. I feel sorry for Mattias that he cannot make the trip to defend his Champion of Champions trophy and shoot for a record fifth title, but hopefully I can do the new-look All Stars team justice.
“Johan is obviously an incredible driver with seven world rallycross titles to his name and I am looking forward to teaming with him in the ROC Nations Cup.
“Saturday is going to be a bit surreal racing head-to-head with legends the likes of Sebastian Vettel, David) Coulthard, Sebastien Loeb and Kurt Busch, but I cannot wait to get out there and give the local Australian crowd some extra to cheer about.
That is one of two-last minute changes to the ROC line-up with third-generation rally ace Max McRae ruled out with a wrist injury.
He will be replaced in the Team GB line-up alongside David Coulthard by his father and former WRC star Alister McRae.
“I’m heartbroken to have to withdraw from the Race Of Champions this year,” said Max McRae.
“We have been keeping an eye on my hand, hoping that it would recover in time.
“Unfortunately I have been advised by doctors that I should not drive until it is healed.
“Since I cannot make it, my father Alister will fill in for me. I’m excited to see what he can do.”
Alister McRae added: “I’m really disappointed for Max. We hoped his injured hand would recover quickly, but unfortunately it’s not there yet.
“I’m obviously happy to be representing Team GB alongside DC, and looking forward to competing and putting on a good show in Sydney on Friday night.”
ROC boss Fredrik Johnsson expressed support for the two drivers that will miss out while welcoming the newcomers.
“It is so disappointing for Mattias and Max, but their health is the number one priority and we wish them the fastest possible recovery,” he said.
“Fortunately we have managed to find two great replacement drivers on short notice in Chaz and Alister.
“Alister and David Coulthard represented Team GB at the ROC Nations Cup at Wembley Stadium in the past and I imagine that the Australian fans will have no issue with another local driver to cheer for, especially one as talented and as popular as Chaz.”
Race Of Champions kicks off this Friday at Accor Stadium in Sydney,
Tickets are still available via Ticketek.
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