Giants interim head coach Steve Spagnuolo says he made the decision, but he added that he discussed the matter with Giants owner John Mara.
And there was even a large group of Giant fans in Oakland pre-game chanting at Manning during warmups "Eli Manning!" and "That's our quarterback!"
Sales of Manning's jersey surged more than 70 per cent after his benching last week, compared with sales over the previous seven days, according to data from sports retailer Fanatics, which runs the National Football League's official online shop.
Smith threw one touchdown pass in his first start for the franchise, but the Giants were beaten 24-17 and sacked head coach Ben McAdoo a day later.
The game also ended Manning's streak of 210 consecutive starts, dating back to 2004.