Thornber, who is the club's head of youth, has taken over after Robins completed his switch to Oakwell on Wednesday.
And Thornber believes the Millers' squad is good enough to cope with the loss and maintain their flying start to the new season.
"We have players here who have come through adversity in the past two or three years and they have always come back stronger and I expect the same this time," Thornber told the Sheffield Star.
"We all had a chat and they were told it would be a real shame if they let it go. But they are all really good pros, there are no bad eggs in there, and they are all focused on what they are trying to achieve.
"I don't think it will be a problem. They want to continue doing well."