George Russell will replace Lewis Hamilton in the Sakhir Grand Prix while Formula 2 driver Jack Aitken will make his Formula 1 debut for Williams.
Despite suggestions that Mercedes reserve driver Stoffel Vandoorne might join Mercedes in place of the seven-time F1 champion, the team has instead opted for Russell.
Williams has released the 22-year-old to compete alongside Valtteri Bottas after team-mate Hamilton tested positive for COVID-19.
Russell has had a long-standing association with Mercedes, joining the junior programme in 2017 whilst racing in the GP3 Series.
With four race wins and seven podiums, Russell won that year’s single-seater series for ART Grand Prix and went on to win the FIA Formula 2 Championship with the same team in 2018.
“Firstly, I want to say a huge thank you to everybody at Williams for giving me this opportunity,” said Russell.
“I might be wearing a different race suit this weekend, but I’m a Williams driver and I’ll be cheering my team on every step of the way.
“I see this as a great chance to learn from the best outfit on the grid right now and to come back as an improved driver, with even more energy and experience to help push Williams further up the grid.
“A big thank you also to Mercedes for putting their faith in me. Obviously, nobody can replace Lewis, but I’ll give my all for the team in his absence from the moment I step in the car.
“Most importantly, I wish him a speedy recovery. I’m really looking forward to the opportunity and can’t wait to get out on track this week.”
Aitken, 25, will join Nicholas Latifi in the Williams camp, marking his first competitive outing in F1 after nearly three full seasons in the F2 feeder series.
As Williams’ official reserve driver, Aitken participated in Free Practice 1 in the Styrian Grand Prix at the Red Bull Ring.
“I’m absolutely over the moon to have the opportunity to make my debut with Williams this coming weekend and I am extremely happy for George to have his chance too,” said Aitken.
“I really mean it when I say I’ve felt very much at home here since I joined Williams earlier this year, so to get my chance to help the team try to achieve that elusive points finish is an extremely satisfying occasion to say the least.
“I’ll be doing all I can to prepare in the coming days, but truthfully, I feel like I have been ready since Melbourne.
“I also want to wish Lewis well in his recovery, and good luck to George getting the chance to drive the Mercedes this weekend.”
This weekend’s Sakhir Grand Prix will get underway with Free Practice 1 on Saturday at 12:30 AEDT.