Kupp is dealing with a "muscle strain on the side of the lower part of his leg right above the foot," Ian Rapoport of NFL Network reports.
There was previous speculation that Kupp had an ankle sprain, but that report was refuted by coach Sean McVay, who relayed that the wideout had some soreness in his leg. Rapoport's report adds a bit of context to that, but more importantly notes that Kupp should be fine for the Rams' season opener.