The team confirmed on Tuesday that recently crowned seven-time Formula 1 champion Hamilton would miss this weekend's Sakhir Grand Prix around the Outer Track of the Bahrain International Circuit.
Mercedes quickly turned to Williams to lure away Russell, a driver the German manufacturer has supported over the years via its junior development programme.
Russell now has the ideal opportunity to prove he is worthy of a permanent seat in the future, potentially as early as 2022 once his three-year deal with Williams expires at the end of next year.
Suggested to Russell this was the moment he had been waiting for, he told GPFans: "Absolutely, but this was not the way I envisioned getting this opportunity, or the way I imagined my first race with Mercedes would be.
"I've mixed feelings because with Lewis obviously having to miss it because of Covid, it is a bit of an odd feeling, but nevertheless a fantastic opportunity.
"I'm incredibly grateful for it to Williams and Mercedes. Williams was under no obligation whatsoever to give me this chance, so I'm just excited."
With two races remaining, and Williams almost certainly set to remain in 10th and last place in the constructors' championship, it was felt they could not stand in his way.
"They recognised this would be a massive opportunity for me personally, and that I would be incredibly grateful and thankful to them for it," added Russell.
"Ultimately, this opportunity will allow me to become a stronger driver, and a new opportunity, new experiences with the best team on the grid is invaluable."
Referring to the Emilia Romagna Grand Prix at Imola when he was running 10th on track late on, only to crash behind the safety car and seemingly throw away a points-scoring opportunity, Russell said: "I guess they had to be realistic.
"We've given it our best shot all season, we've had some fantastic races, and even when all the stars were aligned and we had a great weekend, we were only just P10, and even then it would have been hard to hold on to P10 with the guys behind us.
"Unless we get a P8 in a race [over the last two races] then 10th is sealed in the championship so I think they wanted to support me. They knew how much I wanted this opportunity."
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