According to Kenny Wilkerson (WWL 870), last week Horn was making steady progress and was ready to play Sunday, but he "tweaked" his groin injury during last Friday's workout. He then had to sit out practice. According to Kenny, Payton's rule is that "if you can't practice by Friday, you don't play!"
Horn and Jammal Brown were not making any progress into Saturday and he told them both to stay home and get treatment.
Kenny stated that Joe could have played against the Bucs if he was needed, but Payton implemented his "Friday rule"; therefore Joe and Jammal stayed home.
Kenny mentioned yesterday that Joe should be ready to play this week and would address the issue on his radio show this Thursday night.
Hope that answers your question to some degree. We'll know more this evening after they practice today.
Just a guess but had this game been at home he would have played because Payton would have seen him Sat. Someone that called into WWL said he was golfing early Sun morning so I think he would have at least made a go at it.