Smith spent the 2009 preseason with the Packers, but they re-acquired him in a trade with Jacksonville a couple months ago. He was immediately brought in to improve the special teams, but he hasn't been able to play the past couple of games since sustaining a serious ankle injury. It's nothing new to see another player placed on I.R., but he had only played on special teams and his loss won't be substantial.
They had no choice but to promote Harrell with only WR Jordy Nelson left as the emergency quarterback. Harrell would be overmatched if forced to play on Sunday because it's unlikely he's spent much time with the first team offense. But he played OK during the preseason, and he's as good as anyone else who might be available this late in the season. Since Rodgers failed the concussion protocol, it would be Harrell's turn if anything happened to QB Matt Flynn.