Watts, 24, will sit out the first four games of the 2015 season without pay. The second-year tailback, signed by St. Louis as an undrafted free agent out of Tulsa, will be eligible to return to the active roster on October 5.
Watts is allowed to participate in all off-season activities and pre-season games.
Watts gained 30 yards on seven carries as a rookie and made three catches for 18 yards. He served mostly on special teams, where he made six tackles in 14 games.
The Rams selected a running back, Georgia's Todd Gurley, in the first round of this year's draft.
- Jeremy Woo