The Rams took a calculated risk, choosing to franchise tag safety Lamarcus Joyner rather than receiver Sammy Watkins.
The plan was to continue to pursue Watkins with the hope that demand for his services on the free-agent market would not put him out of reach.
The market has spoken.
Watkins will not return to the Rams, a person with knowledge of the situation said Tuesday, and several published reports said he will sign with the Kansas City Chiefs on Wednesday, when the NFL's new league year begins.
Watkins is expected to sign a three-year, $48-million contract, with $30 million guaranteed, according to NFL.com's Ian Rapoport.
It would have cost the Rams nearly $16 million to put the franchise tag on Watkins, acquired last August in a trade for cornerback E.J. Gaines and a second-round pick in this year's draft.
The Rams saw the trade as an opportunity to add a deep threat, though one with a history of injuries. If Watkins remained healthy and flourished, they planned to possibly sign him to a long-term contract.
If they were unable to do that, they presumed they would receive a third-round compensatory draft pick for losing him as a premium free agent.
Watkins caught 39 passes, eight for touchdowns.