Actually it seems that it was to play a big 3 man front and rush in a controlled fashion.
Davis and rivers are very different players.
Davis is a large DE, a guy who could move inside to dt.
Rivers is a small de, a guy who could be an olb.
While they kind of play the same position they have very different strengths and weaknesses within the overall skill set ofthe position.
The point being that Davis playing week 1 doesn’t necessarily mean they believe Davis is a better de but it means for the game plan against the type of offense they have created unique schemes against he was the best fit for the unique one they used.