The 24-year-old comes into Vern Cotter's starting 15 for the crucial clash against South Africa in the World Cup. Russell injured his right ankle in the win over the United States on Sunday afternoon and he is not expected to return to training until next week.
The pressure is now on Weir to deliver the goods as he tries to help his side qualify for the quarter-finals.
Although he has played just 20 minutes so far, he insists the occasion will not get to him.
"I don't really think about that," he said. "I think about what I can bring as an individual. I think about what I can control.
"There's no point me thinking about outside factors like that. For me, it's about doing my job really well, making sure I boss the team and (am) really vocal in attack. Those things are all in my control and I'll just focus on that."