The Seahawks took a 10-3 lead early in the third quarter, but the Rams recovered to earn a third straight win.
The Rams are now 10-4, level with the Arizona Cardinals atop the NFC West.
In Tuesday’s other rearranged game, quarterback Jalen Hurts made a winning return as the Philadelphia Eagles beat the Washington Football Team 27-17.
Stafford completed 21 of 29 passes for 244 yards in the Rams’ win while Kupp made nine catches for 136 yards and continues to lead the league for receiving touchdowns (14) and receiving yards (1,489).
The Seahawks had won their previous two games but are now 5-9 and are set for a losing record for the first time in quarterback Russell Wilson’s 10 seasons in the NFL.
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Hurts missed one game with an ankle injury, with back-up Gardner Minshew coming in to lead the Eagles to a week 13 win over the New York Jets.
But the second-year quarterback returned against Washington to rush for two touchdowns and throw for another, making 20 of 26 passes for 296 yards.
Miles Sanders gained 131 yards on 18 carries and Dallas Goedert caught seven passes for 135 yards as the Eagles (7-7) moved ahead of Washington on the cusp of the NFC play-off standings.
Washington went 10-0 up but Hurts scored either side of half-time to give Philadelphia a lead they did not let go.
With quarterbacks Taylor Heinicke and Kyle Allen on Washington’s Covid list, much-travelled veteran Garrett Gilbert made just his second NFL start after being signed off the New England Patriots’ practice squad on Friday, making 20 of 31 passes for 194 yards.
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