Following a tweet from Elon Musk that Tesla would announce details on a planned electric semi truck, one analyst is already downgrading its would-be competitors.

Piper Jaffray analyst Alex Potter published a note on Tuesday saying he is downgrading engine and truck manufacturers Cummins and Paccar, in part because because "their valuations already reflect cyclical optimism, but also because we think TSLA's impending arrival could pressure valuations."