Christian, who captains Nottinghamshire, will miss the entirety of the group stage while Derbyshire do not expect McDermott to play for them again in 2021
T20 Blast 2021 team guide: Notts Outlaws
T20 Blast 2021 team guide: Derbyshire Falcons
Nottinghamshire's Dan Christian and Derbyshire's Ben McDermott have pulled out of the T20 Blast on the eve of the tournament after being called up by Australia.
The duo were among six players added to Australia’s preliminary squad ahead of the national side's upcoming white-ball tours of West Indies and Bangladesh.
Both players are returning to Australia to complete two weeks quarantine ahead of possible selection.
Christian, who has won two T20 Blast titles with Notts Outlaws, will miss the entire group stage and will be replaced as captain by Steven Mullaney.
Derbyshire do not expect wicketkeeper-batsman McDermott to play for them again this season.
"This is a huge blow for us, but Ben's been in and around the Australia squad for some time now and his performances have earned him this opportunity," said Derbyshire’s head of cricket, Dave Houghton.
"We've begun enquiries to find a replacement, but it’s difficult at this late stage before the T20 and with a busy international schedule. This is professional cricket and these things happen. Our group of players have the chance to step up and make their mark."
Commenting on his call-up, Christian said: "Getting picked by Australia again is a significant ambition fulfilled for sure, and particularly exciting in such an important year of T20 cricket heading into the World Cup.
"It's disappointing that I won't get to don the Notts jersey this year and get the chance to play alongside those guys again in 2021. It's become a big part of my life in recent times and I'll miss it. But there's certainly enough talent and experience in the group and I'm confident that, if they play well, they can enjoy another successful tournament.”
Australia add six players to West Indies and Bangladesh white-ball squad
Meanwhile, Notts head coach, Peter Moores, added: "It changes things for us late in the day ahead of the tournament, but first and foremost you can only be really pleased for Dan. To be called up again by a nation as strong as Australia, and for that to come on the back of playing some of the best cricket of his career and at the age of 38, it's a great achievement for him.
"It's going to be strange not having Dan around. He's been here for every Blast campaign since 2015 and he's led the side brilliantly as well, but what it will do is create opportunities for other players to step up.
"There is depth in our squad and a huge amount of experience in there as well. We're looking forward to putting on a good show and giving it everything to defend the trophy."
Nottinghamshire begin their title defence against Worcestershire Rapids on June 9 while Derbyshire face Lancashire Lightning