Indianapolis agreed to the deal Saturday.
Herring broke his left forearm on the opening kickoff Aug. 23 against Buffalo and is expected to miss another seven weeks. Sehorn broke his left foot in practice July 27 and is expected to miss at least the first four regular-season games.
"He can line up and play and know what the calls are and know what to do and be involved in the nickel packages," Rams coach Mike Martz said. "I think it was a really important move for us."
For Coady, it's an opportunity to rejoin the team that selected him in the third round of the 1999 draft. Coady spent three seasons in St. Louis, making 69 tackles and intercepting one pass.
Coady had 12 tackles and one interception last season with the Tennessee Titans. After signing with the Colts, Coady played late into preseason games, including Friday night's finale against Cincinnati.
Coady was expected to battle last year's starters, Idrees Bashir and David Gibson for playing time, but the Colts chose safety Mike Doss in the second round. When Bashir and Doss locked up the
starting spots, Coady was relegated to backup duty.
The Colts kept 11 defensive backs after Sunday's final cutdown, but team officials decided Saturday that Coady did not need to be in the mix.
"We had several teams call about safeties, it seems to be that kind of year," coach Tony Dungy said. "Rich was going to be right on the borderline, so we decided we could trade him. St. Louis was
anxious and they did pull the trigger."