Robbie Ray is one of the best strikeout pitchers in baseball. Ray is highly motivated to show well being a free agent at the end of the season. He looked great in spring training. 
I am also high on San Diego's starting pitcher, Dinelson Lamet. He has a tremendous fastball and is two years removed from Tommy John surgery that cost him the 2018 season. Lamet has improved his slider and curveball. He is a major breakout candidate.
Lamet, like Ray, had a dominant spring training. He is going against a Diamondbacks lineup whose bottom third is very weak. 
Playing at Petco Park, the premier pitcher's park, is another plus for the Under.